fix(init): layer the repo's .gitignore into the init index walk

[tiendungdev]

Fixes #324.

Root cause

indexRepoForInit resolved its config with:

cfg, err := config.Load("")

config.Load takes a config file path, not a repo path, so the per-repo
layering never ran and cfg.Index.Exclude came back empty. indexer.New then
hit its own documented fallback:

patterns := idx.config.Exclude
// A nil/empty list from upstream means "no layering was applied"
// (e.g. a direct caller of indexer.New without ConfigManager).
if len(patterns) == 0 {
    patterns = excludes.Builtin
}

cmd/gortex/init.go is exactly that direct caller, so the init walk ran on
excludes.Builtin alone and admitted every gitignored tree that does not happen
to match a builtin pattern. Only the daemon reached the layered list, via
ConfigManager.GetRepoConfig → EffectiveExclude.

The admission path never consults .gitignore on its own either —
dirIgnoreFiles is {".gortexignore", ".ignore", ".rgignore"} — so the
.gitignore layer exists solely in internal/config.

Why this is a bug rather than a design choice

RespectGitignore is documented as defaulting to true. The promise held for the
daemon and silently did not hold for init.

Symptom (from #324)

On a repo whose .gitignore excludes tmp/ (holding four linked worktrees) and
a generated output directory:

	files
tracked set	2,693
queued by init	15,496 (5.7×)

Still parsing at 18m46s. node_modules/ (29.7k) and .next/ (5.1k) were
correctly skipped because they match builtin patterns — which is what made the
failure look inconsistent rather than absent.

Note for #324 readers: the issue guessed that nested git worktrees were being
treated as admissible repos. That turned out to be a red herring —
LinkedWorktreeRoots only reports already-tracked repos. The worktrees under
tmp/ were swallowed simply because tmp/ was never pruned.

Fix

Resolve the config for the actual root through the same path the daemon uses.
The new initRepoConfig helper:

• keeps the previous best-effort fallback when the global config cannot be read,
  so an unreadable ~/.gortex/config.yaml still does not block indexing;
• takes an explicit global path so tests never read the developer's real global
  config.

Tests

cmd/gortex/init_gitignore_test.go:

• TestInitRepoConfigLayersRepoGitignore — gitignored dirs are excluded, and a
  tracked dir is still walked (so the fix cannot be "exclude everything").
• TestInitRepoConfigHonoursRespectGitignoreOptOut — respect_gitignore: false
  still walks ignored trees.

Sabotage-verified: reverting the helper to the old config.Load("") behaviour
makes TestInitRepoConfigLayersRepoGitignore fail; restoring it passes.

Verification

gofmt, go vet and go build clean.

Full-package runs on Windows show six failures — TestReviewCLI_AudienceAgent
(path separators), TestComputePurgePlan* and TestDefaultGlobalConfigPath_*
(they read the real HOME rather than the temp one), and TestMatchRepo_SuffixMatch
(needs a running daemon). I confirmed all six fail identically on the base commit
7de88fb without this change, so they are pre-existing environment coupling and
not introduced here.