feat: add advisory-only host contract v1

[jhuang-tw]

Summary

Adds a separate, versioned, advisory-only host contract for external workflow authorities such as AutoGPT, without changing djobs-native task ownership semantics.

Base is pinned to v0.19.0 commit 46eff8a1227dc256592a237c9a0af8ba7706d528.

New additive surfaces

• djobs contract --schema-major 1 capabilities
• djobs contract --schema-major 1 observation ...
• djobs contract --schema-major 1 receipt ...
• djobs-contract
• djobs-contract-mcp

The standalone MCP exposes only capabilities, observation, and receipt verification. It does not register agents or expose checkpoint / handoff.

Contract guarantees

• authority=advisory
• side_effects=false
• writes_database=false
• SQLite is opened with mode=ro and PRAGMA query_only=ON
• no agent registration, lease recovery, snapshot capture, task claim, task completion, scheduling, or worktree creation
• repository fingerprint / HEAD mismatch fails open
• structured filters run in the SQLite query stage
• typed per-observation provenance, lifecycle status, freshness, evidence hash, ranking signals
• explicit token-budget and truncation metadata
• query/output audit receipt
• ASCII-safe JSON transport for CP950/Windows consoles

Compatibility

Existing djobs.entrypoint, coding MCP, checkpoint, handoff, queue, scheduler, and storage semantics remain unchanged. The public console dispatcher delegates every non-contract command to the established entrypoint.

Schema major 1 is additive within the major; consumers ignore unknown fields, and a new major requires explicit opt-in.

Validation

Focused isolated tests before push: 10 passed.

Added tests cover:

• capability/authority boundary
• exact repository HEAD + feature filtering
• read-only database invariance
• budget truncation and critical-failure disclosure
• receipt hash verification/tamper detection
• CP950-safe Chinese/emoji/path JSON
• runtime JSON Schema validation
• CLI schema negotiation and routing
• golden fixtures

This is a draft until the repository's complete GitHub CI matrix is green.