feat: add Antigravity IDE bridge extension

[Solar2004]

Summary

Add a VS Code extension (hcom-bridge) for Antigravity (Google's VS Code fork) to enable bidirectional messaging between hcom and Antigravity's Jetski agent.

Changes

• src/hooks/antigravity.rs — extension install/verify/remove
• extensions/hcom-bridge/ — VS Code extension source
• src/antigravity_extension/ — embedded extension bundle
• Wired into hcom hooks management (status/add/remove)
• Architecture docs in antigravity-integration.md

[aannoo]

Correctness Blockers

1. Extension does not import path.

   extensions/hcom-bridge/src/extension.ts imports only vscode, fs, HcomClient, JetskiBridge. It then calls path.join(home, '.hcom', 'extensions') in logToFile. tsc rejects this under strict: true, and extensions/hcom-bridge/build.sh runs tsc first, so the TS source cannot be rebuilt. The committed minified bundle src/antigravity_extension/extension.js references the bare path global; on first activation the extension throws ReferenceError: path is not defined. TS source and committed JS bundle have drifted.

2. send --project is placed after --.

   In extensions/hcom-bridge/src/hcomClient.ts's send method, args = ['send', '@<target>', '--', message] is built first, then args.push('--project', project) appends the flag after the separator. hcom's send grammar treats everything after -- as message text; --project foo becomes part of the message. Project scoping through the bridge's send is non-functional. (For reference, listAgents in the same file builds ['list', '--names'] first and then pushes --project — that ordering is legal; bug is send-specific.)

3. listen --project dependency does not exist.

   The extension's spawn method invokes hcom listen --json --name <name> --project <project>. PR feat: project isolation — agents scoped by --project label #49's diff does not modify src/commands/listen.rs. A project-configured bridge spawns with an unknown flag, exits non-zero, and the bridge's restart timer puts it into a 3-second spawn loop.

4. Registration is non-atomic.

   hcomClient.ts::startAgent runs hcom start --name <name> first, then hcom config -i <name> project <project>. Between the two calls the instance row exists with project=NULL. Under PR feat: project isolation — agents scoped by --project label #49's NULL-as-wildcard semantics, messages from any project can target the just-registered peer during that window. The step-2 promise also swallows errors; if step 2 fails the agent stays cross-project with no retry.

5. extensions.json update is an unlocked read-modify-write.

   src/hooks/antigravity.rs reads ~/.antigravity/extensions/extensions.json, parses, mutates, serializes, writes. No file lock or atomic-rename. Concurrent installs or IDE writes corrupt the registry. The entry written also embeds "$mid": 1, which is a VS Code-internal URI marshalling sentinel — undocumented for external use and subject to change.

6. Committed minified bundle creates two sources of truth.

   src/antigravity_extension/extension.js is the esbuild output of the TS under extensions/hcom-bridge/src/ and is embedded via include_str! in src/hooks/antigravity.rs. Per-change workflow requires editing TS, running build.sh, copying the output, and committing both. The two have already drifted (per Blocker 1). cargo build should regenerate the blob or fail; tests should diff the embedded blob against a rebuild.

7. Uses undocumented command APIs despite declaring a typed API.

   extensions/hcom-bridge/src/jetskiBridge.ts calls vscode.commands.executeCommand('antigravity.startNewConversation') and vscode.commands.executeCommand('antigravity.sendPromptToAgentPanel', ...). Neither command id is documented in Antigravity's public surface; precedent for instability includes the agentSessions service-rename incident. Meanwhile, Antigravity's locally installed vscode.d.ts exposes a typed vscode.antigravityExtensibility.sendToAgentPanel({ message, autoSend, files }) API — and the PR's own extensions/hcom-bridge/src/antigravity.d.ts already declares it. The runtime should call the typed API:

   await vscode.antigravityExtensibility.sendToAgentPanel({
       message: `[hcom message from @${sender}]\n\n${text}`,
       autoSend: true
   });

   with the reverse-engineered command path only as a versioned fallback.

8. EXTENSION_VERSION hardcoded in three places.

   A Rust const in src/hooks/antigravity.rs, plus two package.json files. Bumping requires three coordinated edits plus a Rust rebuild. No compatibility detection against Antigravity itself.