feat(opencode): native OpenCode plugin with per-turn injection, write reminders, compaction flush, session-idle gate and pwf tools

New npm package opencode-planning-with-files under
.opencode/packages/opencode-planning-with-files/ (TypeScript, Vitest):

- chat.message appends the framed active plan (plan head, normalized
  progress tail, findings pointer) to every user message, or a
  once-per-turn ambiguity notice when a direct child project carries its
  own live plan
- tool.execute.after appends the progress reminder to write, edit,
  patch, multiedit and apply_patch output
- experimental.session.compacting keeps the plan pointer and attestation
  hash in the compaction context
- event session.idle runs the completion gate in gated mode and
  re-prompts the session with the gate reason through the SDK (Tier 2);
  child sessions are never re-prompted, the shared .stop_blocks and
  .gate_last_ledger counters and PWF_GATE_CAP bound it
- tools pwf_init (root or .planning/<date>-<slug>/, mode autonomous or
  gated with the v3 markers and attestation), pwf_status, pwf_check

The resolver, ambiguity rule, gate decision table, init markers,
attestation check and frame format mirror resolve-plan-dir.sh,
inject-plan.sh, check-complete.sh --gate, init-session.sh and the Hermes
plugin. Each session resolves its plan from its own OpenCode directory.
PWF_PLAN_ROOT fails closed, PLANNING_DISABLED silences every hook.

Also: .opencode/commands/pwf.md and pwf-status.md in OpenCode's command
format, a dogfood entry .opencode/plugins/planning-with-files.ts that
loads the plugin from source when this repository is opened in OpenCode,
.opencode/package.json tracked with the @opencode-ai/plugin dependency,
and a vitest (OpenCode plugin) CI job.

Verified live in OpenCode 1.18.21: plugin loaded, pwf_init/pwf_status/
pwf_check listed, /pwf and /pwf-status listed, a real session message
received the framed plan as a synthetic part.