Give Antigravity a NodeSpace plugin: hooks for project, skill list and refusal #3777

Problem

nodespace skill install gives Antigravity the skill and an instructions block, and nothing else. In the harness scenario run for #3748 (Antigravity CLI 1.3.4, the skill and block as the installer writes them):

• the probe named the project from the workspace folder's name (widget), not from the graph, and said "unknown" for the skills available;
• a skill edited mid-session was not noticed: asked whether anything about how work is done had changed, the session answered "No", citing the earlier read of the standard;
• the session did not reach the case of a task finished elsewhere, so it is not measured for Antigravity; the Codex and Claude Code plugins refuse the next write in it.

Why a plugin is possible

Checked against Antigravity CLI 1.3.4 with a logging hook in a scratch workspace, and the hooks documentation (https://antigravity.google/docs/hooks):

Observed:

• A workspace .agents/hooks.json (and ~/.gemini/config/hooks.json) runs hooks in print mode. The payload carries conversationId, workspacePaths, toolCall.name (run_command) and toolCall.args.CommandLine.
• ANTIGRAVITY_CONVERSATION_ID (the same id) is in the environment of the agent's own shell commands, so the CLI can name a write journal after it, as it does for CODEX_THREAD_ID.
• Hooks are commands, one process per event: the session state has to be kept in a file per conversation, as for Codex (plugins/codex/hook.ts, createSession(host, interval, state) and snapshot() in the shared module).
• PreToolUse must answer with a decision; a hook that prints {} denies the call. PostToolUse must answer exactly {}: any other field makes Antigravity abort the tool call.
• PostToolUse carries the tool call and an error, with no tool output documented. The shared module learns the item being worked on from a command's output (nodespace query run --with-context), so that case needs the hook to run the read itself, or to read the transcript at transcriptPath.
• Antigravity has agy plugin install/validate and can import Claude Code plugins (agy plugin import claude): the packaging format is the open question.

Acceptance criteria

• nodespace skill install antigravity gives Antigravity hooks (or a plugin) in addition to the skill and block; nodespace uninstall removes them.
• A session in a checkout of a known project receives the project, orientation, consent rules and skill list without asking.
• A skill edited mid-session is announced on the next invocation.
• Once the task being worked on is finished elsewhere, the next shell command or file edit is refused with the reason.
• Write attribution works: the session's own writes are not read as foreign (the CLI names the journal after ANTIGRAVITY_CONVERSATION_ID, or the hook rewrites the command).
• Whether the hooks need user trust is settled and the installer says what the user has to do.
• The scenario in scripts/eval/harness/ is run with the plugin, and the Antigravity column compared with the baseline above.

Refs #3748