fix: diff→symbol joins return empty results in multi-repo daemons

[zzet]

Symptom

Every diff- and PR-driven tool silently reported an empty changeset: gortex prs <N> printed risk LOW (score 0.0), Blast radius: 0 changed symbol(s), and all five review priorities at 0.0 for a 10-file PR. The same all-zero result came back from pr_risk, review, review_pack, diff_context, detect_changes, pr_review_context, suggest_reviewers, critique_review, post_review, sibling_diff_context, and the pre-commit prompt. No errors anywhere — the tools just saw nothing.

Two independent root causes, fixed in one commit each.

Root cause 1 — repo-prefix mismatch in the file→symbol join

Multi-repo daemons key indexed file paths as <prefix>/<rel> (gortex/internal/daemon/overlay.go), but git diffs and forge file lists carry repo-relative paths (internal/daemon/overlay.go). Both join choke points fed the relative path straight into the exact-match GetFileNodes lookup:

• Server.changedSymbolsForFiles (forge paths → get_pr_impact, triage_prs, conflicts_prs)
• analysis.MapGitDiff / MapGitDiffWithLines (git hunk paths → the 13 diff-driven handlers above)

Every lookup missed → 0 changed symbols → every downstream axis (callers, community, coverage, flow, security) scored zero.

Fix

• analysis.JoinFileNodes / JoinFilePath: raw lookup first (keeps unprefixed single-repo graphs and already-prefixed input working, never double-prefixes), then retry with <prefix>/<path>.
• MapGitDiff / MapGitDiffWithLines take an explicit repoPrefix; the shared hunk join is extracted into joinHunksToSymbols (the two functions had verbatim-duplicate loops). DiffResult.ChangedFiles keeps diff-relative paths so git pathspec re-joins are unaffected.
• review.Options.RepoPrefix + BuildChangeView threading for the review engine.
• Server.diffJoinPrefix(repoRoot) resolves root → prefix for the MapGitDiff handlers; Server.prJoinPrefix(ctx, repo) resolves the PR-tools prefix from the repo selector, the picked repo root, or the session's cwd-bound repo — the CLI dials the daemon with its working directory, so gortex prs <N> inside a tracked repo needs no explicit selector.
• suggest_reviewers ownership/co-change joins use the same prefix-aware lookup.

Root cause 2 — git diff header prefixes vs. user config (0891ac8d)

The diff parsers anchor on +++ b/ headers. With diff.mnemonicPrefix=true in the developer's git config, worktree-side diffs emit c/ w/ prefixes (diff.noprefix=true drops them entirely) — every parsed hunk drops, and the unstaged / staged / all scopes report an empty changeset while compare keeps working (commit-to-commit diffs keep a/ b/). The test fixture already pinned both keys to false to protect itself from exactly this; production never did.

Replace the first two "Out of scope" bullets with a third fix section:

Root cause 3 — no unified repo-scope resolution (f391e8fb)

detect_changes / diff_context / the pre-commit prompt defaulted to repoRoot := "." (the daemon's own cwd) via the nil-in-multi-repo single-indexer field; the review family resolved --repo only as a tracked prefix, so the CLI's path default failed; and resolveRepoFilter mis-read the review tools' scope argument as a saved-scope name and the path-form repo as a workspace filter — even a resolvable root then errored.

Fix: Server.diffRepoScope(ctx, repo) — explicit selector (prefix or path, normalized, honoured exclusively), then the lone tracked repo, then the session's cwd-bound repo. All diff-driven handlers use it; detect_changes, diff_context, and the pre_commit prompt grow an optional repo argument; the git-scope values are reserved words to the saved-scope filter.

Fix: analysis.GitDiffArgs builds the diff argv with -c diff.mnemonicPrefix=false -c diff.noprefix=false ahead of the subcommand, and the three previously-duplicated argv builders (diffmap, review old-side rawDiff, sibling per-file diff) now share it.