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]>

Cargo.lock

@@ -4365,9 +4365,9 @@ dependencies = [
 
 [[package]]
 name = "openssl"
-version = "0.10.75"
+version = "0.10.76"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "08838db121398ad17ab8531ce9de97b244589089e290a384c900cb9ff7434328"
+checksum = "951c002c75e16ea2c65b8c7e4d3d51d5530d8dfa7d060b4776828c88cfb18ecf"
 dependencies = [
  "bitflags 2.11.0",
  "cfg-if",
@@ -4403,9 +4403,9 @@ checksum = "7c87def4c32ab89d880effc9e097653c8da5d6ef28e6b539d313baaacfbafcbe"
 
 [[package]]
 name = "openssl-sys"
-version = "0.9.111"
+version = "0.9.112"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "82cab2d520aa75e3c58898289429321eb788c3106963d0dc886ec7a5f4adc321"
+checksum = "57d55af3b3e226502be1526dfdba67ab0e9c96fc293004e79576b2b9edb0dbdb"
 dependencies = [
  "cc",
  "libc",