Add CONTRIBUTING.md, docs/architecture.md, and per-provider docs (#284)

Document the contributor onboarding path:
- CONTRIBUTING.md: setup, npm scripts, coding conventions, PR process,
  the block-claude-coauthor enforcement, and the five providers without
  test coverage today (claude, gemini, goose, qwen, antigravity).
- docs/architecture.md: 12-command CLI surface, parser pipeline, three
  cache layers, 14 optimize detectors, and the mac / gnome / build
  layouts with cited line numbers.
- docs/providers/: one file per provider (17 providers plus the shared
  vscode-cline-parser helper). Each covers data path, storage format,
  caching, dedup key, quirks, and a "when fixing a bug here" checklist.

Also fix two pre-existing documentation issues surfaced while writing
the new docs:
- RELEASING.md claimed GitHub Actions auto-publishes the CLI when a
  v* tag is pushed. There is no such workflow; CLI publishing is
  manual via npm publish. Updated the CLI section to reflect reality
  and kept the menubar (mac-v* tag) automation accurate.
- .gitignore had CLAUDE.md unanchored, which on case-insensitive
  filesystems also matched docs/providers/claude.md. Anchored to
  /CLAUDE.md so the root-level memory file stays ignored without
  affecting subdirectory docs.

All cited file paths, line numbers, function names, and test counts
were verified against current code (41 test files, 558 tests passing).

docs/providers/opencode.md

@@ -0,0 +1,36 @@
+# OpenCode
+
+OpenCode (sst/opencode).
+
+- **Source:** `src/providers/opencode.ts`
+- **Loading:** lazy (`src/providers/index.ts:59-75`)
+- **Test:** `tests/providers/opencode.test.ts` (558 lines, the largest provider test)
+
+## Where it reads from
+
+Default `~/.local/share/opencode/` or `$XDG_DATA_HOME/opencode/`. The discovery walk picks up `opencode*.db` files (`opencode.ts:71-88`).
+
+## Storage format
+
+SQLite.
+
+## Caching
+
+None.
+
+## Deduplication
+
+Per `<sessionId>:<messageId>` (`opencode.ts:242`).
+
+## Quirks
+
+- **Schema validation is loud.** When a required table is missing, the parser logs an actionable warning telling the user which table is gone and what version of OpenCode it expects (`opencode.ts:104-131`). This is the right behavior; do not silently swallow these.
+- Source paths are encoded as `<dbPath>:<sessionId>` (`opencode.ts:147-150`).
+- Each message's `parts` are indexed (`opencode.ts:177-191`); preserving the order matters for reasoning-token correctness.
+- Tokens are reported across `input`, `output`, `reasoning`, `cache.read`, and `cache.write`. Anthropic semantics.
+
+## When fixing a bug here
+
+1. The 558-line test suite catches a lot. Run `npm test -- tests/providers/opencode.test.ts` before and after any change.
+2. If the bug is "missing table" warning, do not catch and silence it. Either upgrade the version expectation in the parser or document the breaking schema change.
+3. If the bug is "reasoning tokens off by one", check the parts index ordering.