feat(res-to-affine): Phase-1 migration assistant skeleton (Refs #57)

[hyperpolymath]

Summary

Lands the .res→.affine migration assistant proposed in #57 as a tight Phase-1 cut:

• tools/res-to-affine/ — OCaml CLI built by the repo's dune toolchain. Text-scans a .res source for 4 of the 6 anti-patterns from idaptik's Wave 3 pilot (side-effect imports, %raw blocks, untyped exceptions / Promise.catch, mutable globals via :=) and emits a .affine skeleton with migration markers + the quoted original at the bottom for reference.
• editors/tree-sitter-rescript/ — manifest-only vendoring of the canonical rescript-lang/tree-sitter-rescript grammar, pinned at commit 990214a (v6.0.0, MIT). Not used yet — wired up in Phase 2.
• docs/MIGRATION-ASSISTANT.adoc — architectural decision: canonical grammar, three-phase plan, alternatives considered.
• tools/res-to-affine/test/ — alcotest snapshot suite. Synthetic fixture exercises all four Phase-1 patterns; spot-checked against gitbot-fleet/bots/sustainabot/bot-integration/src/*.res (correctly finds 0 in Config.res/Webhook.res/Oikos.res, 2 in Main.res, 3 in GitHubAPI.res).

This PR uses Refs not Closes for #57 — the proposal is multi-phase and this is Phase 1 only. Phase 2 (tree-sitter AST walker) and Phase 3 (partial translation of pure-structural forms) are scoped in tools/res-to-affine/README.md and the ADR.

Cross-references: hyperpolymath/gitbot-fleet#148 (consumer — 2,133 LOC ReScript subtree blocked on #57).

Why Phase-1 is text-scan, not tree-sitter

The user picked tree-sitter integration as the long-run direction (right per LESSONS.md "tree-sitter as canonical grammar source"), but committing all of that in one PR would leave no shippable artefact behind if Phase 2 stalls. Phase 1 is deliberately small and useful in isolation:

• Runs against any .res file today.
• Detects 4 of 6 anti-patterns reliably (regex-based; the other two need real AST).
• Same Emitter interface Phase 2 will use, so the swap is local.
• Gates the architectural commitment behind something that already pays its way.

See docs/MIGRATION-ASSISTANT.adoc for the full rationale and the alternatives considered (bs-tools typed AST, hand-rolled OCaml parser, pattern-detector-only).

Test plan

• dune build clean repo-wide (no regression to existing 80+ modules under lib/).
• dune test tools/res-to-affine/test/ — 3/3 OK (snapshot, scanner kinds, module-name derivation).
• Run against gitbot-fleet .res files end-to-end: scanner output matches expectations (clean files report 0, files with try/Js.Exn/Promise.catch report the right line numbers).
• CI (this PR) — first run on push.

Files

docs/MIGRATION-ASSISTANT.adoc                          NEW  130 lines  ADR
editors/tree-sitter-rescript/README.md                 NEW   46 lines  vendoring manifest
editors/tree-sitter-rescript/package.json              NEW   17 lines
editors/tree-sitter-rescript/scripts/install.sh        NEW   38 lines  +x
tools/res-to-affine/README.md                          NEW  117 lines  usage + Phase plan
tools/res-to-affine/dune                               NEW   11 lines
tools/res-to-affine/main.ml                            NEW   77 lines  cmdliner CLI
tools/res-to-affine/scanner.{ml,mli}                   NEW  ~120 lines
tools/res-to-affine/emitter.{ml,mli}                   NEW  ~100 lines
tools/res-to-affine/test/{dune,test_emit.ml}           NEW   80 lines
tools/res-to-affine/test/fixtures/sample.res           NEW   30 lines
tools/res-to-affine/test/expected/sample.affine        NEW   60 lines  snapshot

Co-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com