fix: use local time instead of UTC for session titles, log filenames, and trace timestamps

[v4ever1luv]

Issue for this PR

Closes #30347

Type of change

• Bug fix

What does this PR do?

Replaces new Date().toISOString() calls with local-time-aware formatting in 3 files so that session titles, log filenames, and dev trace timestamps reflect the system's local timezone instead of always UTC.

Root cause: Date.prototype.toISOString() is defined by ECMAScript to always return UTC (ending with Z), ignoring TZ env var and OS timezone. For users outside UTC, this causes session titles to show incorrect times.

Changes:

1. packages/core/src/util/log.ts — Added localStamp() and localTime() helpers that use local date components. Log filenames now use local time (e.g. 2026-06-02T070000.log), log entries show local timestamps.

2. packages/opencode/src/session/session.ts — Added toLocalISO() that returns ISO 8601 with timezone offset. Session titles now show local time (e.g. New session - 2026-06-02T07:00:00.000+07:00). Updated isDefaultTitle regex to match both old UTC (Z) and new local (+/-HH:MM) formats for backward compatibility.

3. packages/opencode/src/cli/cmd/run/trace.ts — Updated stamp() to use local date components; trace event times use toLocaleString() instead of toISOString().

How did you verify your code works?

• Verified all 3 files compile within their respective packages
• Tested regex change against both old (Z) and new (+07:00) title formats
• Changes are purely mechanical: replace UTC-generating calls with equivalent local-time-generating calls, same output format but in local time

Screenshots / recordings

N/A — not a UI change (affects log files, session titles in database, and dev trace files)

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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