feat(heartbeat): fire_at time-of-day scheduling with IANA timezone (#1029)

* feat(heartbeat): fire_at time-of-day scheduling with IANA timezone support

- HEARTBEAT_FIRE_AT=HH:MM — fire heartbeat at a specific time of day instead
  of on a rolling interval; format is 24h HH:MM (e.g. "14:00")
- HEARTBEAT_TIMEZONE=Region/City — IANA timezone name for fire_at (e.g.
  "Pacific/Auckland", "America/New_York"). Defaults to UTC.
- When fire_at is set, interval_secs is ignored
- Config also readable from settings.toml [heartbeat] section

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

* feat(heartbeat): wire fire_at + timezone into HeartbeatConfig runner

Missed file from heartbeat scheduling commit. HeartbeatConfig struct in
agent/heartbeat.rs now carries fire_at: Option<NaiveTime> and timezone: Tz
so the runner can schedule against a fixed time of day.

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

* fix: add chrono-tz dependency for heartbeat fire_at timezone support

The chrono-tz crate was used in the heartbeat fire_at commits but
its Cargo.toml entry was lost during rebase conflict resolution.

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

* style: rustfmt fix for chained method call

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

* test(heartbeat): add fire_at scheduling and DST safety tests

- test_default_config_has_no_fire_at: interval-based default unchanged
- test_with_fire_at_builder: builder sets time and timezone
- test_duration_until_next_fire_is_bounded: result always 1s–24h
- test_duration_until_next_fire_dst_timezone_no_panic: US Eastern DST
- test_resolved_tz_defaults_to_utc: missing timezone falls back to UTC
- test_resolved_tz_parses_iana: IANA string resolves correctly

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

* fix(heartbeat): restore drift-free interval, add settings.json fallback for fire_at

- Interval path: restore tokio::time::interval (drift-free) instead of
  tokio::time::sleep which drifts by loop body execution time
- fire_at config: fall back to settings.heartbeat.fire_at when
  HEARTBEAT_FIRE_AT env var is not set, consistent with other settings

Addresses Gemini Code Assist review feedback.

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

---------

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

src/config/heartbeat.rs

@@ -7,17 +7,19 @@ use crate::settings::Settings;
 pub struct HeartbeatConfig {
     /// Whether heartbeat is enabled.
     pub enabled: bool,
-    /// Interval between heartbeat checks in seconds.
+    /// Interval between heartbeat checks in seconds (used when fire_at is not set).
     pub interval_secs: u64,
     /// Channel to notify on heartbeat findings.
     pub notify_channel: Option<String>,
     /// User ID to notify on heartbeat findings.
     pub notify_user: Option<String>,
+    /// Fixed time-of-day to fire (HH:MM, 24h). When set, interval_secs is ignored.
+    pub fire_at: Option<chrono::NaiveTime>,
     /// Hour (0-23) when quiet hours start.
     pub quiet_hours_start: Option<u32>,
     /// Hour (0-23) when quiet hours end.
     pub quiet_hours_end: Option<u32>,
-    /// Timezone for quiet hours evaluation (IANA name).
+    /// Timezone for fire_at and quiet hours evaluation (IANA name).
     pub timezone: Option<String>,
 }
 
@@ -28,6 +30,7 @@ impl Default for HeartbeatConfig {
             interval_secs: 1800, // 30 minutes
             notify_channel: None,
             notify_user: None,
+            fire_at: None,
             quiet_hours_start: None,
             quiet_hours_end: None,
             timezone: None,
@@ -37,6 +40,19 @@ impl Default for HeartbeatConfig {
 
 impl HeartbeatConfig {
     pub(crate) fn resolve(settings: &Settings) -> Result<Self, ConfigError> {
+        let fire_at_str =
+            optional_env("HEARTBEAT_FIRE_AT")?.or_else(|| settings.heartbeat.fire_at.clone());
+        let fire_at = fire_at_str
+            .map(|s| {
+                chrono::NaiveTime::parse_from_str(&s, "%H:%M").map_err(|e| {
+                    ConfigError::InvalidValue {
+                        key: "HEARTBEAT_FIRE_AT".to_string(),
+                        message: format!("must be HH:MM (24h), e.g. '14:00': {e}"),
+                    }
+                })
+            })
+            .transpose()?;
+
         Ok(Self {
             enabled: parse_bool_env("HEARTBEAT_ENABLED", settings.heartbeat.enabled)?,
             interval_secs: parse_optional_env(
@@ -47,6 +63,7 @@ impl HeartbeatConfig {
                 .or_else(|| settings.heartbeat.notify_channel.clone()),
             notify_user: optional_env("HEARTBEAT_NOTIFY_USER")?
                 .or_else(|| settings.heartbeat.notify_user.clone()),
+            fire_at,
             quiet_hours_start: parse_option_env::<u32>("HEARTBEAT_QUIET_START")?
                 .or(settings.heartbeat.quiet_hours_start)
                 .map(|h| {