feat: multi-repo checks via origin-to-local-checkout mapping (--repo flag + [[repos]] config)

[laulauland]

What

Origin-qualified anchors (origin = "github:owner/repo") are currently always reported SKIP when they don't match the current repo. This stack lets a consumer repo map an origin to a local sibling checkout, so those anchors get checked for real: fingerprints recomputed against the neighbor's files, blame from the neighbor's git history.

drift check --repo github:fiberplane/drift=../drift

or persistently:

# .drift/config.toml
version = 1

[[repos]]
origin = "github:fiberplane/drift"
path = "../drift"

Stack (one commit per concern)

1. feat(config): load .drift/config.toml — extracts the lockfile's TOML-subset machinery into src/toml.zig (behavior-preserving: existing lockfile tests unchanged) and adds src/Config.zig with strict parsing, line-numbered diagnostics, missing-file-is-default. No user-visible change.
2. feat(check): --repo flag — src/repo_map.zig (parseSpec/validate/immutable RepoMap), lint resolution at the origin-mismatch branch, blame against the mapped root, new mapped_repo_missing skip reason (a teammate without the sibling checkout degrades to SKIP, never an error). Integration tests run against two real temp git repos.
3. feat(config): [[repos]] mapping — config entries validated by the same validate as flag specs, buildMerged with CLI-wins precedence; config paths resolve against the lockfile root (checkout-location independent), flag paths against cwd. DECISIONS.md §15 documents the precedence choice.

Each commit leaves zig build test green; the tip relinks drift.lock for the changed bound files (SKILL.md's origin section updated to document the mapping; CLI.md/DESIGN.md were updated within the stack).

Notes

• Based on fix(parse): zig symbol anchors never resolved — query targeted wrong grammar node names #33 (zig symbol-anchor fix) — merge that first; this PR's diff includes it until then.
• Semantics: a mapped foreign anchor reports fresh/stale exactly like a local anchor. Whether cross-repo staleness should be a softer state (BEHIND) is deliberately out of scope.
• Tracked as DRIFT-dthoathc (subissues DRIFT-jfjdaiic, DRIFT-alslylrl, DRIFT-cvtstpuc).