feat: parse Pi and Antigravity sessions (fixes #46)

[felipetruman]

Description

Adds session parsers for two more agents, registered as external-harness collectors alongside Claude Code, Codex, and OpenCode:

Pi (badlogic/pi-mono)

• Reads JSONL sessions from ~/.pi/agent/sessions/<encoded-cwd>/<timestamp>_<uuid>.jsonl
• Full fidelity: user/assistant turns, per-turn token usage summed across agentic steps (including cacheRead/cacheWrite, same convention as the Claude Code parser), model ids, tool calls, edited/referenced file paths, elapsed time, and workspace root from the session header's cwd

Antigravity (Google)

• Reads per-conversation SQLite databases from ~/.gemini/antigravity-cli/conversations/*.db via the node:sqlite builtin (Node >= 22.5, skipped gracefully when unavailable — same approach as fix: parse OpenCode sessions from SQLite storage (opencode.db) #170)
• Step payloads are undocumented protobuf blobs; a minimal wire-format decoder (~70 lines, no dependencies) reads the reverse-engineered fields: prompt text, response/thinking text, tool calls with JSON arguments, token usage, and timestamps. The field map is documented in the module header. Every access is best-effort, so schema drift degrades to missing data instead of a crash
• Workspace roots resolved from the sibling cache/last_conversations.json map

Both harnesses are added to EXTERNAL_HARNESS_SET, hasExternalHarnessSources(), and the extension's marketplace description.

Related Issues

Fixes #46 — together with #170 (OpenCode SQLite storage), all three requested agents (antigravity, pi, opencode) now load.

Checklist

• npm run check passes (typecheck + lint + spellcheck + knip + tests) — 1218 tests, tsc --noEmit clean, ESLint 0 errors and 0 new warnings in the parser files, cspell 0 issues, knip clean
• Changes are covered by tests (parser-pi.test.ts: turn assembly, usage summing, no-request/no-usage sessions; parser-antigravity.test.ts: synthetic conversation db with hand-encoded protobuf payloads, plus a no-user-message db)
• Documentation updated (module headers document both layouts incl. the reverse-engineered protobuf field map; marketplace description lists the new harnesses)

Verification against real data

• Pi: 66 sessions / 379 requests parsed from a real ~/.pi install; 377 requests carry token data; models, tools (bash, read, edit, write), and file paths resolve
• Antigravity: 164 conversations / 232 requests parsed from a real ~/.gemini/antigravity-cli install; 212 requests carry token data, 200 carry response text; workspace roots resolve via last_conversations.json; locked/empty databases are skipped without error

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