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/agent/thread_ops.rs

@@ -746,6 +746,16 @@ impl Agent {
             let mut job_ctx =
                 JobContext::with_user(&message.user_id, "chat", "Interactive chat session");
             job_ctx.http_interceptor = self.deps.http_interceptor.clone();
+            // Prefer a valid timezone from the approval message, fall back to the
+            // resolved timezone stored when the approval was originally requested.
+            let tz_candidate = message
+                .timezone
+                .as_deref()
+                .filter(|tz| crate::timezone::parse_timezone(tz).is_some())
+                .or(pending.user_timezone.as_deref());
+            if let Some(tz) = tz_candidate {
+                job_ctx.user_timezone = tz.to_string();
+            }
 
             let _ = self
                 .channels
@@ -1111,6 +1121,8 @@ impl Agent {
                     tool_call_id: tc.id.clone(),
                     context_messages: context_messages.clone(),
                     deferred_tool_calls: deferred_tool_calls[approval_idx + 1..].to_vec(),
+                    // Carry forward the resolved timezone from the original pending approval
+                    user_timezone: pending.user_timezone.clone(),
                 };
 
                 let request_id = new_pending.request_id;