cleanup: dead code, F401/F841 enforcement, Z-timestamp bug fix

[johnnichev]

First cut of the post-1.0 code-cleanup backlog (#150). Everything here is internal/private — no public symbol is touched, the 1.0 frozen API is unchanged. Net -32 lines across 61 files.

Closes #135, #136, #137, #138. Advances #149.

Bugs fixed

• Knowledge stores raise on Z-suffixed timestamps (Python 3.10); consolidate ISO-parse into a shared _time.py #136 — knowledge & checkpoint loaders crashed on Z-suffixed timestamps on Python 3.10. datetime.fromisoformat did not accept a trailing Z (UTC designator) until Python 3.11, so a value like "2026-06-15T12:00:00Z" — exactly what Postgres/Supabase/Redis return — raised ValueError on the project's minimum interpreter. All 17 ISO-parse sites now route through one selectools._time.parse_iso helper that normalizes the suffix first. New tests/test_time.py locks in the regression (Z suffix, fractional seconds, offsets, naive, datetime passthrough).
• selectools status: CLI computes a status string then prints the wrong variable #135 — selectools doctor dropped the status text it computed. The per-key line built a set / not set string and then printed only the OK / MISSING icon. Now shows both (OPENAI_API_KEY: OK (set)), matching the deps-section style.

Hygiene

• Remove confirmed dead code (unused serve dataclasses, 2 dead helpers, 5 dead locals) #137 — removed confirmed dead code: 3 unused serve dataclasses (InvokeRequest, BatchRequest, BatchResponse), 2 unreachable private helpers (bm25._score_document, hybrid._find_matching_key), 4 dead locals, and 104 unused imports across src/. Each verified to have zero call sites before removal.
• Tighten ruff config: enable F841, scope the F401 ignore to __init__.py #138 — tightened ruff: F401 (unused imports) and F841 (unused locals) are now enforced everywhere; the F401 ignore is scoped to **/__init__.py re-export hubs. Dead imports/locals can no longer accumulate silently — this is what surfaced the 104 + 4 above.
• Deprecate & remove the vestigial PineconeVectorStore(environment=...) param #149 (partial) — documented PineconeVectorStore(environment=...) as deprecated/ignored (Pinecone v3+ infers the host). The parameter removal stays a breaking change scheduled for a later minor.

Verification

• Full suite: 7650 passed, 159 skipped, 0 failed, 0 errors (Python 3.11).
• mypy src/selectools clean (182 files including the new _time.py).
• ruff check + ruff format --check clean; stability audit map unchanged.

Notes

The DRY consolidation is deliberately scoped to the one item that also closed a bug (the timestamp helper). The larger DRY items in the backlog (#139 LLM-judge scaffold, #140 session validators, #141 import guards, etc.) are left as separate follow-ups.

Changelog entry is under [Unreleased]; this is release-ready as a 1.0.1 patch whenever you want to tag it.