feat(mcp): add scheduled tasks tools

[cchristous]

Summary

Adds three new MCP tools for managing Semaphore scheduled tasks (periodics):

• tasks_list: List scheduled tasks for a project with pagination
• tasks_describe: Get task details including recent trigger history
• tasks_run: Trigger a task to run immediately

Why These Changes Are Safe

Security Controls

• UUID validation: All ID parameters validated via regex patterns at schema level and handler level
• RBAC enforcement: project.scheduler.view required for list/describe, project.scheduler.run_manually for run
• Feature flag gating: Read operations gated by mcp_server_read_tools, write operations by mcp_server_write_tools
• Path traversal protection: pipeline_file parameter rejects absolute paths, .. sequences, backslashes, and control characters
• Parameter validation: Task parameters must start with letter/underscore, no control characters, max 128 char names

Implementation Safety

• Follows established patterns from existing MCP tools (jobs, pipelines, workflows)
• All proto getters use nil-safe Get*() methods
• Explicit nil checks on proto messages and slice elements
• Context timeouts on all gRPC calls via CallTimeout()
• Response truncation prevents oversized outputs

Test Coverage

• 7 unit tests covering:
  • Feature flag disabled scenarios
  • Successful list/describe/run operations
  • Missing required parameters
  • Invalid UUID validation
  • Write feature flag for run tool

Files Changed

File	Purpose
pkg/tools/tasks/*.go	Tool implementations (list, describe, run)
pkg/internal_api/periodic_scheduler/*.go	gRPC client stubs
pkg/internalapi/config.go	Scheduler endpoint configuration
pkg/internalapi/manager.go	Provider interface + Manager
test/support/*.go	Test mocks and stubs
Makefile	Proto generation modules
README.md	Tool documentation

Test plan

• Unit tests pass (go test ./pkg/tools/tasks/...)
• Full test suite passes (make test)
• Lint passes (make lint)
• Manual testing with MCP client against staging environment
• Verify RBAC denies access without proper permissions
• Verify feature flags properly gate tool availability