feat(agent): agent-native wave phase 2 — fetch handler, read_state, commands bridge, git agent tools

[antfubot]

Why

Phase 2 of plan 031 (stacked on #142). The MCP endpoint was h3-only, shared state was invisible to tools-only MCP clients, hub commands had no path to the agent surface, and the git plugin's read RPCs — the most obviously agent-useful data in the workbench — weren't exposed.

What

• createMcpFetchHandler(ctx, options) — the MCP Streamable-HTTP endpoint as a framework-agnostic web-standard Request → Response handler (sessions, origin gate, teardown included). mountMcpHttp is now a thin h3 wrapper over it. This is the enabler for serving MCP from inside non-h3 hosts (@devframes/next, phase 3).
• Built-in read_state tool — tool-shaped shared-state access (no key → key list, key → JSON value), honoring the same exposeSharedState filter as the resource projection. Many MCP clients only consume tools; the resources stay for those that don't.
• Hub commands → agent bridge — opt-in agent field on server commands (same default-deny convention as defineRpcFunction): description-as-prompt, safety (default action), optional positional valibot args (single object unwrapped). Projections track register/update/unregister. agent on a handler-less group throws the new DF8404 (docs page included); the field never crosses the wire.
• devframe/utils/valibot-json-schema — the valibot→JSON-Schema conversion promoted from an MCP-adapter internal to a public util, so SDK-free hosts (the hub) can convert schemas.
• Schema-typed handlers may be async — the schema-typed definition branch now types handlers as Thenable<InferReturnType<RS>> (explicit RS ⇒ no inference risk; sync handlers stay assignable). This unblocks schema-typing genuinely-async RPCs — the limitation plan 001 previously hit.
• git agent surface — status/log/show/branches/diff agent-flagged with args/returns schemas (safety: 'read' inferred). Write ops stay agent-invisible. Cross-refs plan 029.

Verification

Full gate: pnpm lint && pnpm test && pnpm typecheck && pnpm build — 896 tests (10 new: read_state list/read/filter/deny, commands-bridge projection/re-sync/zero-arg/DF8404/wire-stripping); API snapshots updated deliberately.

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This PR was created with the help of an agent.

[antfubot]

Folded into #145 together with phase 3 — the phase-3 branch already contains these commits, so #145 now targets the phase-1 branch and carries both phases.