feat(opencode): add lightweight project memory

[LeXwDeX]

Summary

• add worktree-local lightweight MEMORY with fixed YAML topics and project/global JSONC configuration
• add /memory on|off, hidden-model maintenance, main-session-only prompt injection, and pre-compaction checkpoints
• reject code, plans, documentation, secrets, and unconfirmed transient content before persistence
• keep MEMORY files out of Git through repository-local exclusions

Verification

• Ask Matt Standards review: no findings
• Ask Matt Spec review: no findings
• pre-commit: 4,852 warnings / 0 errors (ratchet met); 29/29 package typechecks passed
• MEMORY regression: 77 passed / 1 existing skip; prompt integration: 2 passed
• DAG core gate: Core 90, OpenCode 402, Schema 3, TUI 50; all 16 line/function coverage floors passed
• generated SDK: no diff
• OpenSpec strict validation: passed

Notes

• existing untracked .opencode/** artifacts are not included
• local Bun workspace discovery reports CouldntReadCurrentDirectory on this checkout; repository fallback gates and the PR Typecheck workflow provide the canonical verification