feat(opencode): add pluggable log rotation strategies

[Haoping-Xiao]

Closes #12960

What

Adds pluggable write strategies to Log.init so log rotation behavior is configurable without changing default behavior.

Two strategies:

• startup (default) — existing behavior, one file per startup, retain last 10 files
• daily — one file per day (opencode-YYYY-MM-DD.log), same-day restarts append, retain 7 days by default

How

Log.Options gets two new optional fields:

path?: string    // custom log directory
rotate?: Rotate  // { kind: "startup" | "daily", retention?: number }

A writeStrategies record maps each kind to its init function. Adding a new strategy = add a type variant + a function. init() dispatches in one line:

return writeStrategies[options.rotate?.kind ?? "startup"](options)

Env vars for configuration (since Log.init runs before Config loads):

• OPENCODE_LOG_PATH — override log directory
• OPENCODE_LOG_ROTATE=daily — enable daily rotation
• OPENCODE_LOG_RETENTION=14 — override retention count

Not passing anything = zero behavior change.

Verification

• Ran bun run typecheck — passes
• Tested startup strategy (default) — same file naming and cleanup as before
• Tested OPENCODE_LOG_ROTATE=daily OPENCODE_LOG_PATH=/tmp/opencode-logs — generates opencode-2026-02-10.log, appends on restart
• Tested retention cleanup with 10 dummy files + OPENCODE_LOG_RETENTION=3 — correctly keeps newest 3

[github-actions]

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No duplicate PRs found