feat: set_presence REST endpoint, MCP tool, and shared PresenceStatus enum

[tlongwell-block]

Summary

Add PUT /api/presence so GUI clients and agents can set their presence status without requiring a WebSocket connection. Introduces a shared PresenceStatus enum in sprout-core as the single source of truth, used by both the relay REST API and the MCP agent surface.

Implements the set_presence MCP tool that was explicitly deferred in #28:

set_presence MCP tool (requires new REST endpoint or WS event dispatch)

What Changed

Shared Types (sprout-core)

• PresenceStatus enum (Online, Away, Offline) defined once in sprout-core::presence
• schemars::JsonSchema derived conditionally via mcp-schema feature flag — zero overhead for non-MCP consumers
• Includes Display, as_str(), PartialEq/Eq, and serde rename_all = "lowercase"
• 4 unit tests: serde roundtrip, unknown variant rejection, as_str, Display

REST API (sprout-relay)

• PUT /api/presence — accepts {"status": "online" | "away" | "offline"}
  • Returns {"status": "...", "ttl_seconds": N} (90 for online/away, 0 for offline)
  • "offline" calls clear_presence() (Redis DEL); others call set_presence() (Redis SET EX 90)
  • Auth via extract_auth_pubkey — users can only set their own presence
• ApiJson<T> custom extractor — wraps axum::Json<T> to return standard {"error": "..."} envelope on deserialization failure, preserving Axum's original status codes (400/415/422)

MCP Tools (sprout-mcp)

• set_presence tool (fix: just reset race condition #41) with typed PresenceStatus enum in the JSON schema — gives LLM callers a tight, validated schema instead of a free-form string
• Error propagation improved — RelayClient GET/POST/PUT/DELETE/get_api now include status code, URL, and response body in error messages

TESTING.md

• Tool count updated 40 → 41 throughout
• Test counts updated: 40 REST, 14 relay, 14 MCP = 68 total
• Presence tools table updated (1 → 2 tools)
• Exercises A-5 and B-7 updated to use set_presence
• Expected results table updated with presence verification row

Testing (9 new e2e tests + 4 unit tests)

• 5 REST tests: set online (+ TTL), away→offline transitions (+ TTL + status), invalid status (422 + error envelope), auth required (401 + body), missing field (422 + envelope)
• 4 MCP tests: set online (+ TTL), offline (+ TTL), away (+ TTL), invalid status (JSON-RPC error)
• All 68 integration tests pass (40 REST + 14 relay + 14 MCP)
• cargo fmt, cargo clippy, cargo check all clean

Files Changed (13 files, +560/-30)

File	Change
crates/sprout-core/src/presence.rs	New — PresenceStatus enum, single source of truth
crates/sprout-core/Cargo.toml	schemars optional dep, mcp-schema feature
crates/sprout-core/src/lib.rs	pub mod presence, pub use PresenceStatus
crates/sprout-mcp/Cargo.toml	Enable mcp-schema feature on sprout-core
crates/sprout-mcp/src/server.rs	set_presence tool using shared enum
crates/sprout-mcp/src/relay_client.rs	Error body propagation (all 5 HTTP methods)
crates/sprout-relay/src/api/presence.rs	set_presence_handler with TTL response
crates/sprout-relay/src/api/mod.rs	ApiJson<T> custom extractor
crates/sprout-relay/src/router.rs	GET(...).PUT(...) on /api/presence
crates/sprout-test-client/tests/e2e_mcp.rs	4 new tests, tool count 41, JSON-parsed assertions
crates/sprout-test-client/tests/e2e_rest_api.rs	5 new tests with TTL + error envelope verification
TESTING.md	Updated tool count, test counts, exercises, expected results
Cargo.lock	Updated lockfile

Design Notes

• WebSocket path (kind:20001) is unchanged — it still accepts arbitrary status strings for forward-compatibility. The REST/MCP surface intentionally restricts to the curated PresenceStatus enum. This divergence is documented in the handler doc comment.
• PUT is the correct verb — presence is a single-field resource being fully replaced.
• TTL in response — returning ttl_seconds lets clients know when to send the next heartbeat without hardcoding the value.
• ApiJson<T> preserves Axum's original status codes while wrapping rejections in the standard error envelope, so 415 (wrong Content-Type), 400 (malformed JSON), and 422 (unknown enum variant) all get the right status code with a consistent body format.
• Feature flag design — schemars is optional in sprout-core behind mcp-schema, keeping the zero-I/O-dependency principle intact for non-MCP consumers.

Test Results

test result: ok. 40 passed; 0 failed; 0 ignored   ← REST API
test result: ok. 14 passed; 0 failed; 0 ignored   ← relay
test result: ok. 14 passed; 0 failed; 0 ignored   ← MCP

All 68 integration tests pass. 4 sprout-core unit tests pass.