feat(sync): add daemon-trusted stable-root routing (TIN-2853)

[Jesssullivan]

Summary

Adds the narrow daemon-trusted root identity seam required to inspect and resolve an isolated repo conflict cache without accepting client-supplied state paths or storage prefixes.

Changes

• Enrolls stable root IDs to trusted local-root, remote-prefix, state-cache, and inspect/resolve policy records.
• Adds daemon-selected conflict inspection and a dedicated registered-root keep-both RPC, so mixed-version fallback cannot mutate the primary cache.
• Fences legacy rootless paths, path/prefix overlap, cache and manifest keys, state ownership/inodes/hardlinks, Git topology, and concurrent state writers.
• Neutralizes repo-controlled Git hooks, fsmonitor, reflog routing, and ambient Git routing variables before attended resolution.
• Adds CLI route evidence, source-bounded protocol documentation, changelog coverage, and an ADR for the live ceremony that remains outstanding.
• Registered roots inherit the one global storage endpoint, CA, credentials, and TLS policy.

Test plan

• Rust formatting and clean-diff checks
• Scoped all-target check and strict scoped Clippy
• 45 focused core, sync, daemon, CLI, MCP, TLS, and FileProvider tests
• Independent exact-head adversarial review: no P0/P1 findings
• Signed commit and clean worktree verified

Checklist

• Docs updated
• CHANGELOG.md updated
• No secrets or credentials in diff
• Source-only claim boundary recorded

Claim boundary

This PR does not enroll or deploy a live root, verify an authentication session, execute a live dry-run or mutation, restart reconciliation, or prove multi-cycle fleet convergence. The active incident freeze remains in force.

Linear: TIN-2853

Packaging/runtime Git availability remains a separate blocker under TIN-2688; this source seam does not claim installed-service readiness.

[Jesssullivan]

Independent Linux Nix repair proof:

• nix build .#packages.x86_64-linux.tcfsd --no-link --refresh --option eval-cache false completed successfully on the configured x86_64 Linux builder.
• The build exercised the real-Git registered-root safety tests with nativeCheckInputs = [ pkgs.git ].
• That invocation captured pre-rebase source head d376381d385a01cc8b5d6e8dbaeeb24875440b10; the two-commit patch was then signature-preserving rebased without conflict onto cleanup merge f4cb680897f3f42da4c93a7b9b7b74f08118d571.
• Current exact head 69dfe079670eebd9cd7e3c24aa92a529f11ebd27 is still gated on its fresh GitHub CI.

No fleet, auth, deployment, or live root state changed.

[Jesssullivan]

Exact-head adversarial review found blocking root-identity and compatibility/claim gaps in the local hardening follow-up. Returning this PR to draft while those findings are fixed and revalidated; the incident freeze and source-only boundary remain unchanged.