plan(ssh): bake in resolved decisions + ~/.ssh/config discovery

Update the SSH support plan to reflect user-confirmed decisions and a key
simplification from a new requirement:

- New §0.5 'Resolved decisions' records all 7 confirmed answers.
- Computer is now a READ-ONLY view over ~/.ssh/config (Host aliases), not a
  persisted CRUD entity: no computer-store package, no create/update/delete
  API. computerId IS an SSH config alias. ~/.ssh/known_hosts is the host-key
  trust store (auto-trust-and-pin).
- Auth simplified to key-only from ~/.ssh (no gopass/SecretsAccess/secretRef
  anywhere).
- ssh2 only (no bun-ssh2 fork); verifying under Bun is the load-bearing
  Phase-3 first step.
- LSP/MCP silently dropped on remote turns (no system-prompt note);
  edit_file works with no diagnostics on remote.
- computerId persisted per-conversation (like cwd).
- Updated data model (§3), connection mgmt (§4), security (§7), edge cases
  (§8), API surface (§9 read-only), frontend (§10), packages table (§11,
  no computer-store), phases (§12), and resolved open questions (§13).

Planning document only; no code changed. No merge or push.

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