feat(indexer): git-aware untracked-asset skip + bounded init store

[zzet]

Summary

Two follow-ups to #181 (content-admission caps), both targeting the #120 OOM from a different angle. Stacked on #181 — base is fix/index-content-admission-caps; review/merge after it (retarget to main once #181 lands).

Motivation came from the reporter's actual intake.json: the 4.42 GiB admitted corpus is untracked RAG scratch data, and the OOM happens on gortex init, which indexed entirely in RAM.

A — opt-in skip of untracked non-source assets (index.skip_untracked_assets)

respect_gitignore is already on, but .gitignore can't catch files that are merely untracked (never git added, not ignored) — which is exactly how uncommitted RAG corpora / datasets / build outputs get admitted.

• New index.skip_untracked_assets (default off, per request — current behaviour is unchanged unless enabled).
• When on, a full-index walk builds the repo's git ls-files set once and drops asset-class files (document/data/image) git doesn't track, with an untracked_asset skip node.
• Untracked CODE is still indexed, so new/unsaved source keeps working.
• Inert on a non-git repo or when git ls-files is unavailable (everything admitted as before). Applies to the cold full-index walk; the incremental watcher path keeps the size/class caps only.

For the reporter: flipping this on removes the entire untracked 4.42 GiB with zero per-file tuning.

D — gortex init indexes into a temporary on-disk store

indexRepoForInit used graph.New() (all-in-memory), with no content sink — so document/section text was never leaned and the whole post-parse graph was pinned in RAM. That's the path the reporter's OOM most likely hit (parsing got 2040 files … killed).

• Init now indexes into a temporary sqlite store: nodes persist per file and the content sink leans section text to disk, so init inherits the daemon's shadow / byte-budget guards.
• The store + temp dir are removed once skill generation / coverage have read the graph.
• Both consumers (genskills.Build, parity.CoverageOf) already take graph.Store, so the change is confined to indexRepoForInit and its two callers.

Validation

• go build ./... (CGO) — success
• go test -race ./internal/indexer/ ./internal/config/... — 768 passed (incl. new untracked-asset tests with a real git repo)
• go test ./cmd/gortex/ -run 'Init|Parity|Wire|Intake' — 47 passed
• go vet + golangci-lint on changed packages — clean
• Smoke: built the binary, ran gortex init on a temp git repo → succeeds, 0 leftover temp stores (cleanup verified)

Relates to #120.