feat: archive submitted feedback to the shared plannotator index - #56

Implements plannotator-tui's side of the shared feedback archive (Plannotator PR backnotprop/plannotator#1438, schema v1 frozen at 443e1fcf).

On every successful send (agent delivery or clipboard), one record is appended to {PLANNOTATOR_DATA_DIR}/feedback/{project}/index.jsonl with client: plannotator-tui, plus a -plannotator-tui-suffixed markdown sidecar for content-bearing records.

Contract points implemented:

• lock-free O_APPEND whole-line append: one write call (write_all banned), Interrupted-only retry, terminate-don't-splice on partial, archiving never fails the send
• surfaces annotate / annotate-folder / annotate-last; target.agent {host, session, transcript} for agent-message reviews; clientVersion stamped
• origin in Plannotator's spellings (claude-code, copilot-cli, oh-my-pi)
• opt-out parity: PLANNOTATOR_FEEDBACK_HISTORY env over feedbackHistory in the data dir's config.json over default-on
• sidecar exclusive-create with collision counter, written before the index line

Pinned-byte test locks the exact v1 line shape; further tests cover collisions, decision-only lines, env/config knob semantics, host label mapping, and agent provenance.

DO NOT MERGE until Plannotator's #1438 merges (contract freeze) — the ops session confirms the merged sha first.

https://claude.ai/code/session_01L232F8ev8RX6Jwd7PepsUz