feat(timezone): add timezone-aware session context (#671)

* feat(timezone): add timezone-aware session context (#661)

All timestamps were UTC-only, causing daily logs to split at UTC midnight,
cron schedules to fire in UTC, and no quiet hours for heartbeat. This adds
timezone as a per-session property flowing from the client.

Key changes:
- New `src/timezone.rs` module with resolution chain, parsing, and detection
- `IncomingMessage` carries optional timezone from client
- `JobContext.user_timezone` flows timezone to tools
- `next_cron_fire()` accepts timezone for schedule evaluation
- `Trigger::Cron` stores optional timezone (backward-compatible)
- Workspace gains `_tz` variants for daily logs and system prompt
- Heartbeat supports quiet hours (`HEARTBEAT_QUIET_START/END`)
- Web frontend sends `Intl.DateTimeFormat().resolvedOptions().timeZone`
- REPL auto-detects system timezone
- `DEFAULT_TIMEZONE` env var / settings for server-wide default

Storage stays UTC. Conversion happens at display boundaries.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* fix(timezone): address review feedback on timezone-aware sessions

- Validate quiet hours values (0-23) in HeartbeatConfig::resolve()
- Fall back to settings values when env vars are unset for quiet hours
- Validate IANA timezone strings in routine_create/update with parse_timezone
- Add timezone field to routine_create tool schema
- Allow standalone timezone update on cron routines without changing schedule
- Return path from append_daily_log_tz to avoid TOCTOU race at midnight
- Delegate append_daily_log to append_daily_log_tz(entry, UTC) to avoid drift
- Preserve timezone through approval flow via PendingApproval.user_timezone
- Improve test_today_in_tz to not depend on hardcoded year
- Add 3 regression tests for quiet hours config validation

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* style: fix formatting in routine.rs

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* fix(timezone): address second round of review feedback

- Remove .claude/scheduled_tasks.lock from repo and add to .gitignore
- Store resolved timezone (not raw message.timezone) in PendingApproval
- Carry forward user_timezone through chained approvals in thread_ops
- Wire quiet_hours_start/end from config to HeartbeatRunner
- Support X-Timezone header as fallback in chat_send_handler

[skip-regression-check]

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* fix(timezone): include user's local time in time tool response

The time tool's "now" operation now returns local_iso and timezone
fields based on ctx.user_timezone, so the LLM can report time in
the user's timezone instead of always UTC.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* style: fix formatting in time.rs

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* fix(timezone): address Copilot review round 3 — validation, deterministic tests, schema fixes

- Validate DEFAULT_TIMEZONE and HEARTBEAT_TIMEZONE at config load time
- Add timezone field to HeartbeatSettings and config::HeartbeatConfig
- Wire heartbeat timezone from config through agent_loop to HeartbeatRunner
- Add timezone to routine_update tool schema (was accepted but not advertised)
- Error on schedule/timezone update for non-cron routines
- Validate timezone in Trigger::from_db (coerce invalid to None with warning)
- Validate timezone in approval path (thread_ops.rs) before overwriting
- Time tool always includes timezone/local_iso fields (fallback to UTC)
- Make quiet hours tests deterministic using current UTC hour
- Add regression tests for config validation

[skip-regression-check]

Co-Authored-By: Claude Opus 4.6 <[email protected]>

---------

Co-authored-by: Claude Opus 4.6 <[email protected]>

src/timezone.rs

@@ -0,0 +1,110 @@
+//! Timezone resolution and utilities.
+
+use chrono::{DateTime, NaiveDate, Utc};
+use chrono_tz::Tz;
+
+/// Resolve the effective timezone from a priority chain.
+///
+/// Priority: client_tz > user_setting > config_default > UTC
+pub fn resolve_timezone(
+    client_tz: Option<&str>,
+    user_setting: Option<&str>,
+    config_default: &str,
+) -> Tz {
+    // Try each in priority order, skipping invalid values
+    for candidate in [client_tz, user_setting, Some(config_default)] {
+        if let Some(tz) = candidate.and_then(parse_timezone) {
+            return tz;
+        }
+    }
+    Tz::UTC
+}
+
+/// Parse a timezone string (IANA name) into a `Tz`.
+pub fn parse_timezone(s: &str) -> Option<Tz> {
+    s.parse::<Tz>().ok()
+}
+
+/// Get today's date in the given timezone.
+pub fn today_in_tz(tz: Tz) -> NaiveDate {
+    Utc::now().with_timezone(&tz).date_naive()
+}
+
+/// Get the current time in the given timezone.
+pub fn now_in_tz(tz: Tz) -> DateTime<Tz> {
+    Utc::now().with_timezone(&tz)
+}
+
+/// Detect the system's timezone, falling back to UTC.
+pub fn detect_system_timezone() -> Tz {
+    iana_time_zone::get_timezone()
+        .ok()
+        .and_then(|s| parse_timezone(&s))
+        .unwrap_or(Tz::UTC)
+}
+
+#[cfg(test)]
+mod tests {
+    use chrono::Datelike;
+
+    use super::*;
+
+    #[test]
+    fn test_resolve_client_wins() {
+        let tz = resolve_timezone(Some("America/New_York"), Some("Europe/London"), "UTC");
+        assert_eq!(tz, chrono_tz::America::New_York);
+    }
+
+    #[test]
+    fn test_resolve_user_setting_fallback() {
+        let tz = resolve_timezone(None, Some("Europe/London"), "UTC");
+        assert_eq!(tz, chrono_tz::Europe::London);
+    }
+
+    #[test]
+    fn test_resolve_config_fallback() {
+        let tz = resolve_timezone(None, None, "Asia/Tokyo");
+        assert_eq!(tz, chrono_tz::Asia::Tokyo);
+    }
+
+    #[test]
+    fn test_resolve_all_none_utc() {
+        let tz = resolve_timezone(None, None, "UTC");
+        assert_eq!(tz, Tz::UTC);
+    }
+
+    #[test]
+    fn test_resolve_invalid_client_skipped() {
+        let tz = resolve_timezone(Some("Fake/Zone"), Some("Europe/London"), "UTC");
+        assert_eq!(tz, chrono_tz::Europe::London);
+    }
+
+    #[test]
+    fn test_parse_valid() {
+        assert_eq!(
+            parse_timezone("America/Chicago"),
+            Some(chrono_tz::America::Chicago)
+        );
+    }
+
+    #[test]
+    fn test_parse_invalid() {
+        assert_eq!(parse_timezone("Fake/Zone"), None);
+    }
+
+    #[test]
+    fn test_detect_system_tz() {
+        // Should always return a valid Tz (at minimum UTC)
+        let tz = detect_system_timezone();
+        let _ = now_in_tz(tz); // Should not panic
+    }
+
+    #[test]
+    fn test_today_in_tz_returns_valid_date() {
+        let date = today_in_tz(Tz::UTC);
+        // Verify it returns a valid date (year, month, day are all positive)
+        assert!(date.year() > 0);
+        assert!((1..=12).contains(&date.month()));
+        assert!((1..=31).contains(&date.day()));
+    }
+}