refactor(agent): drop two verified-dead symbols (vulture, #1136)

[axisrow]

Part of #1136 (vulture axis of epic #1130). Two symbols deleted; each verified dead across all five surfaces (FastAPI/Web/CLI/agent/TUI) before removal. Per #1136, "dead by graph" ≠ "unneeded" — so every candidate below passed a full grep + git-history check before deletion.

Deletions

1. src/agent/tools/_registry.py — require_args() (8 lines)

• Repo-wide grep (src/ tests/ scripts/ docs/ conftest.py, across .py/.txt/.md/.rst): exactly 1 hit — the definition. No string references, no __all__ export, no wildcard import of _registry (the repo has zero import \* statements), no getattr dispatch by that name.
• git history: born in refactor(agent): add shared tool context helpers #534 (refactor sharing tool-context helpers) already without a caller or test — git show 0fdae381 | grep -c require_args → 1 (definition only). Dead since birth.
• Five surfaces: it's a private helper inside the agent-tool registry; tools are reached via the registry, never via this function.

2. src/agent/provider_registry.py — ZAI_BASE_URL_REQUIRED_HINT constant (5 lines)

• Repo-wide grep: 1 hit — the definition. The hint text ("Z.AI Base URL is optional…") is not duplicated anywhere else.
• git history: born used in fix(agent): Z.AI Coding Plan support — required base_url + 1113/1309/1310/1311 hints #528 (Z.AI Coding Plan support: assigned to _init_error, returned from a check, and raised in a RuntimeError). The last consumer was removed in refactor(agent): canonicalize provider metadata #535 (canonicalize provider metadata) — git grep ZAI_BASE_URL_REQUIRED_HINT f49ce429 → definition only. Orphaned leftover of that refactor.
• Sibling ZAI_* constants (ZAI_GENERAL_BASE_URL, ZAI_CODING_BASE_URL, ZAI_DEFAULT_BASE_URL, ZAI_LEGACY_ANTHROPIC_BASE_URLS) remain — they are used.

Verification (no regression guard possible — code is gone, so proof = green suite + this note)

• ruff check src/agent/tools/_registry.py src/agent/provider_registry.py — clean.
• python -c "import src.agent.tools._registry, src.agent.provider_registry" — OK.
• Full suite (pytest tests/ -m "not aiosqlite_serial" -n auto then -m aiosqlite_serial) — green (the only unrelated failures are pre-existing on origin/main: tests/test_quality_scoring_service.py and the xdist-flaky tests/routes/test_debug_routes.py::test_debug_timing_page; both deselected and reproduced on clean main).

Not deleted in this PR (doubtful → left for owner decision)

• src/agent/tools/_photo_loader_runtime.py::resolve_photo_target_id — 0 production callers (last call migrated to resolve_photo_target in fix(agent): correctness fixes incl. confirm-bypass (837#1,#7,#5, 838#10, 835#10) #842), but its docstring explicitly calls it a "Backward-compatible shim". Intentional compat surface → not deleted without explicit owner "yes".

Part of #1136 / #1130.