feat: Phase 1 advisory leases and structured safety evaluator

[toasterbook88]

This PR unifies the Phase 1 advisory resource leases (Triangle spec) and the structured safety evaluator into a single robust security layer.

Changes Implemented

1. Advisory Resource Leases (Triangle):

   • Added triangle_request_lease, triangle_release_lease, and triangle_heartbeat_lease MCP tools.
   • Wired syscall.Flock exclusive file locking on ~/.axis/ledger.json.lock in internal/reservation/persist.go with a 500ms timeout retry loop to ensure concurrency safety.
   • Propagated call context (context.Context) through the locking path to ensure retries are gracefully cancellable.
   • Standardized lease ID structure (lease-<UnixNano>) and enforced duration_seconds > 0.
   • Integrated automatic reservation ledger reloading in internal/daemon/daemon.go.

2. Structured Safety Evaluator:

   • De-gated the structured safety evaluator by deleting the compile-gating stubs in structured_default.go and removing the //go:build safety_scaffolded tag from structured.go.
   • Unified safety gates by delegating VerdictDeny commands in safety.Check (blocker.go) directly to the structured ruleset, preserving existing cluster-resource fallbacks.
   • Added TestCheckDelegatesToStructuredEvaluator in blocker_test.go and imported strings.

3. Hygiene & Documentation:

   • Resolved 8 linter warnings regarding unused parameters in mcp_client.go, explain.go, and triangle.go.
   • Added comprehensive design documentation including docs-evaluation.md, future/triangle-orchestration.md, and the hybrid spec.

Verification Results

• All unit and integration test suites compile and pass successfully (go test ./... -> PASS).
• Executions of blocked commands (e.g. sudo rm -rf) and prompted commands behave correctly under the unified gate.

[gemini-code-assist]

Code Review

This pull request introduces the 'Triangle' multi-agent orchestration substrate, adding advisory resource lease tools (triangle_request_lease, triangle_release_lease, triangle_heartbeat_lease) to the MCP server and integrating cross-process file locking in the reservation ledger. It also wires the structured safety evaluator into the execution blocker and removes obsolete compile-gated stubs. Feedback on these changes suggests making the lease duration parameter truly optional by applying a default value, and optimizing the safety check path by reusing a package-level evaluator instance instead of recreating it on every call.

[toasterbook88]

I have addressed the review feedback and updated the PR:

1. Optional Lease Duration: Updated triangleRequestLeaseTool in internal/mcp/triangle.go so that if duration_seconds is <= 0, it defaults to 120 seconds (2 minutes). Also updated the corresponding test suite in internal/mcp/triangle_test.go (TestTriangleTools_DefaultDuration) to verify this fallback behavior.
2. Safety Evaluator Optimization: Declared a package-level defaultEvaluator in internal/safety/blocker.go and reused it inside Check(), preventing the overhead of re-allocating the ruleset and evaluator on every call.

All local quality checks pass. Pushed updates to trigger CI validation.