D194 — Support Google Antigravity in multi-provider client and provisioning

antigravity is added as a supported provider in the client configurator and provisioning engine (D137). It configures Google Antigravity (agy) across client discovery, MCP server emission, standing instructions, skills, subagents, and native hooks.

Context. Antigravity is an agentic coding assistant that communicates with MCP servers and loads project/global instructions and skills. It was previously unsupported by Cartographer, requiring manual configuration of .gemini/config/mcp_config.json, standing instructions in .gemini/GEMINI.md, and skills under .gemini/config/skills/.

Every destination below was verified against Google's own documentation rather than inferred: MCP (https://antigravity.google/docs/cli/mcp/), hooks and their five events (https://antigravity.google/docs/hooks/), skills (https://antigravity.google/docs/skills/), subagents (https://antigravity.google/docs/subagents/) and global rules (https://antigravity.google/docs/rules-workflows/). These are external contracts that change outside this project's release cycle: re-verify before editing the matrix.

Decision. - Registry and Detection (D137). Registered ProviderAntigravity Provider = "antigravity" with DisplayName: "Antigravity". Detection probes the Antigravity CLI executable agy, the Antigravity app bundle on macOS, and Antigravity-specific configuration directories. The legacy Gemini CLI executable gemini and a generic leftover .gemini/ directory are deliberately not evidence: they would produce false positives after the two products diverged. - MCP Configuration Emission. Emits .gemini/config/mcp_config.json with top-level key mcpServers. HTTP transport emits serverUrl and optional headers. Header values (such as Bearer ${CARTOGRAPHER_TOKENS}) are passed through verbatim: Antigravity natively resolves ${VAR} environment variables. Stdio transport emits command, args, and env. DeletableWhenEmpty: true allows pruning the configuration file and its parent directory if removing Cartographer leaves no foreign servers behind. - A file Cartographer cannot parse is refused, never normalized. An earlier revision of this work accepted JSONC (line/block comments, trailing commas) in Antigravity's own files by stripping them lexically before decoding. Rejected: the rewrite path serializes with json.MarshalIndent, so every comment the user wrote would be deleted on the first sync — silently, and on a file Cartographer does not own. Antigravity's documented format is plain JSON; the contract is therefore the same one every other provider has had since D1 ("existing file %s is not valid JSON — fix or delete it manually"), and the file is left byte-for-byte untouched when it cannot be parsed. A uniform refusal is better than a provider-specific lossy repair. - Provisioning Destination Matrix. - mcp: .gemini/config/mcp_config.json (shared JSON configuration, per-server entries) - instructions: .gemini/GEMINI.md (managed marker-delimited instructions block) - skill: .gemini/config/skills/<name>/ (directory materialized with SKILL.md and assets) - agent: .gemini/config/agents/<name>.md, translated to Antigravity frontmatter (name, description, mainAgent: false, subagent: true) while preserving the prompt body - hook: files in .gemini/config/hooks/<name>/, registered as an owned cartographer-<name> definition in .gemini/config/hooks.json; supported native events are PreToolUse, PostToolUse, PreInvocation, PostInvocation, and Stop - Trigger semantics, declared as a property and not as a provider check. Antigravity has native hooks but no session-start event: its five events all fire per tool call or per model invocation, so mapping SessionStart onto one of them would run sync on every turn — a different behaviour wearing the same name. The hook mechanism therefore declares noSessionStartEvent, and SupportsSessionHook derives the answer from it. KB hooks are installed and registered normally; the generated bootstrap hook (D60) is not installed, and connect/status name the scheduled timer (D140) exactly as they do for the other providers without a session hook. The property is deliberately data on the mechanism rather than an if provider == …: the next provider whose hook engine lacks the event declares it too. - Mount Point Preservation. .gemini is registered in provisioningRootDirs so pruneEmptyDirs never deletes the user's root Antigravity configuration directory on disconnect or skill pruning.

Known divergence, reported and not repaired. ~/.gemini/GEMINI.md is not exclusively Antigravity's: Gemini CLI writes its own global context to the same path (https://github.com/google-gemini/gemini-cli/issues/16058), and Antigravity additionally reads a cross-tool ~/.gemini/AGENTS.md. Cartographer's managed block is marker-delimited and non-destructive, so coexistence in the file is safe, but the precedence between the two files is the provider's, not ours. This is the same class of problem as D189 (an instructions file written correctly and never read): it belongs to the shadowing detection that plan introduces, and is recorded here so that work covers Antigravity from the start rather than discovering it later.

Consequences. - cartographer agents detects installed Antigravity instances. - cartographer connect antigravity, connect all, and the interactive connect form configure Antigravity. - cartographer disconnect antigravity prunes managed artifacts and removes MCP entries without residue, leaving the root .gemini/ directory intact. - cartographer status and sync track drift and synchronize skills, subagents, hooks, MCP endpoints, and instructions. - An Antigravity config carrying comments now fails connect/sync with a named error instead of being rewritten. This is the intended behaviour, and the message says what to do. - Field testing found two incompatibilities after release (#273): the server refused Antigravity's notifications over their Accept header (D200), and Antigravity drops tools whose qualified identifier exceeds 64 characters (D201).