fix(plugin): bound all session.messages() calls with limit:50

The legacy OpenCode session messages endpoint hydrates the ENTIRE
session into memory when no query.limit is provided. On huge sessions
(10k+ messages) — exactly when Magic Context's compaction shines —
this defeats the plugin's purpose by loading all persisted messages
into RAM just to extract the latest assistant text from a helper
subagent.

All 9 production callsites only need:
- The latest assistant message from a helper subagent (historian,
  compressor, dreamer, sidekick, key-files, user-memory) — 1 user
  prompt + a few assistant steps typically <10 messages.
- Recent tail of the main session for conflict-warning cleanup —
  scanned backwards from the end, breaks on first non-warning user.

Both fit comfortably in limit:50 with massive headroom. Pi-side uses
sessionManager.getBranch() directly and has zero session.messages()
calls — no Pi change needed.

Added session-messages-bounded.test.ts as a static source-scan
regression guard so future callsites without `limit` are caught at
test time with a clear error message pointing at the offending file.

Reported by the OpenCode agent during cross-team session.messages
audit. Verified plugin 1428 tests, Pi+CLI 371 tests, typecheck clean.