Complete secure protocol endpoint dispatch

[codeacme17]

Closes #5.

Summary

• route authenticated loopback requests through runtime-validated protocol handlers
• derive advertised capabilities from installed handlers only
• validate request and response schemas, exact opaque IDs, UTF-8 bodies, endpoint-specific response envelopes, and sanitized Remote hosts
• deduplicate concurrent identical commands and reject command-ID collisions before invoking product behavior
• record recoverable operation results before handler invocation and validate recovery responses
• authorize native actions only for exact target/action bindings issued by the latest validated snapshot and execute through one atomic handler seam
• redact schema-valid diagnostic fields before serialization while preserving valid response shapes

Scope

This completes the protocol and security boundary without implementing Repository Git behavior. Product handlers can now be installed behind the validated dispatch seam.

Out of scope

• Repository and Worktree discovery: Discover registered Worktrees with generation-safe identity #6
• refresh, process cancellation, operation lanes, and reconciliation: Implement versioned refresh, operation lanes, and reconciliation #7
• UI and Git product behavior: Build the adaptive Repository and Worktree overview #8 through Add exact-target navigation and optional Codex provenance #15

No known pre-existing defects were found in the touched protocol paths.

Verification

• 86 tests across 16 test files
• authenticated HTTP regressions for validation, command collision/retry, recovery, native authorization, redaction, UTF-8, and supported response limits
• npm run typecheck
• npm run lint
• npm run build
• npm run format:check
• independent Standards and Spec reviews: no findings

PR preflight was skipped at user request.