fix: address 6 merge-blocking issues — exception normalization, label PATCH, §6 direction, artifacts, test hermeticity, state semantics

1. Exception normalization: github_request() now wraps URLError into
   RepoArchitectError alongside HTTPError, so all GitHub API transport
   failures propagate through the same error contract. Tests verify both
   error types produce RepoArchitectError deterministically.

2. Existing-issue label application: added set_github_issue_labels() that
   PATCHes labels on existing issues via the GitHub Issues API. The
   existing-issue update path now calls this after posting the re-scan
   comment, so labels_applied truly means applied — not just computed.

3. dependency_contract.json §6 alignment: reversed layer_order and
   allowed_direction to match charter §6 (runtime → core → knowledge →
   agents → interface). Lane 5 detection removed the false-positive
   agents→interface check (allowed per §6) and added interface→runtime
   and agents→runtime (prohibited per §6.1). New test confirms
   agents→interface is NOT flagged.

4. Generated artifacts removed: .agent/ and docs/repo_architect/issues/
   added to .gitignore; tracked files removed with git rm --cached.

5. Dedupe-failure test hermeticity: test now patches github_request via
   unittest.mock instead of hitting the real GitHub API. Added separate
   tests for URLError and HTTPError normalization in github_request.

6. State-reporting nits: dedupe_result on creation failure changed from
   "new" to "create_failed". _MAX_VIOLATIONS_DISPLAY promoted to
   module-level constant. IssueAction.dedupe_result docstring updated
   to list all valid states.

Co-authored-by: Steake <530040+Steake@users.noreply.github.com>

Author: Copilot