propose: add uv support alongside pip workflow

[HumanBean17]

Summary

Add uv as a first-class developer-workflow option alongside the existing pip install -r requirements.txt path. Neither is removed; the two ecosystems coexist with explicit non-coupling.

Locked decisions

#	Decision
1	uv coexists with pip; nothing is removed
2	requirements.txt stays hand-maintained and authoritative for pip installs — it is NOT regenerated from uv export
3	uv.lock is the authoritative lockfile for uv sync installs; committed to repo
4	The two lockfiles are independent and may legitimately diverge on transitive pins; CI does not enforce coherence
5	Dev deps (pytest, ruff, pytest-asyncio, unidiff) live in [dependency-groups.dev] in pyproject.toml
6	No [tool.uv] table — uv defaults apply
7	No CI changes (no .github/workflows/ exists today)

Full list: 12 locked decisions in §7.

Why two-track instead of single-source-of-truth

The current requirements.txt is a battle-tested freeze (144 pinned transitive deps including cocoindex==1.0.0a43, the LanceDB / sentence-transformers / tree-sitter-java triplet). Re-resolving it via uv's SAT solver risks shifting transitive pins in ways that would need re-validation against the test suite and runtime behaviour on pre-release deps.

The cost of "two artefacts to maintain" is bounded — uv contributors edit pyproject.toml's ranges, pip contributors edit requirements.txt's pins, neither workflow consumes the other's output — and far smaller than an unscheduled dependency-resolver migration.

Blast radius (from §3 surface + §6 plan)

Site	Change
pyproject.toml	Add [dependency-groups.dev] table
uv.lock	New file, generated by uv lock, committed
README.md §1	Add parallel "Install with uv" subsection; document two-track policy explicitly
requirements.txt	No change
.gitignore	No change (.venv/ already on line 7)
.github/workflows/	No change (doesn't exist)

Net: 3 files modified, 1 new file. PR-2 (optional CONTRIBUTING.md) deferred.

What this deliberately does NOT do

10 explicit out-of-scope items in §4 including: delete requirements.txt, regenerate it from uv export, add CI for pip↔uv equivalence, migrate to uv-only, switch build backend.

Use-case re-walk

15 scenarios in §5. Three (UC3, UC5, UC11) surface the same accepted cost: contributors adding/bumping a runtime dep must edit both pyproject.toml (for uv) and requirements.txt (for pip). Documented as developer responsibility in the README install subsection.

Open for review

Status: draft. Per the propose-doc-author skill, this is the iterate-and-amend stage — comments inline on the file, force-push amendments on the same commit, status flips to under review on first feedback and locked once merged.