perf(grouping): return file indices instead of paths

[lizhengfeng101]

Why

The grouping LLM call asked the model to echo full file paths back in its JSON response, so the output size grew with the sum of all path lengths. On large change sets this overflowed the completion token limit; the truncated JSON then failed to parse and the whole change set degraded to per-file dispatch, multiplying downstream review calls. This is the main reason grouping was slow on big diffs.

What

Backend — grouping contract switched from paths to integer indices

• buildFileList prefixes each file with a zero-based index, e.g. [0] MODIFIED path (+12/-3).
• groupingResponse.Files is now []int; the model returns those indices, e.g. [{"label":"...","files":[0,1]}].
• parseGroupingResponse maps indices back to diffs by position, skipping out-of-range indices (the index equivalent of the previous unknown-path skip) and duplicates. A parse failure still returns an error and the caller falls back to per-file dispatch, exactly as before.
• Prompts updated to ask for integer indices.

The response is now an order of magnitude smaller, so truncation on large change sets becomes rare instead of common.

Viewer — resolve indices back to paths for display

Because the grouping response is now indices, the session viewer renders them as paths:

• buildGroupingIndex scans the request's numbered file list (user message only) to build an index→path map.
• parseGroupingGroups unmarshals the response into indices; a legacy path-string response reports not-ok so the viewer keeps showing the raw text (already readable for those older sessions).
• groupingView maps indices to paths and falls back to the raw response when nothing resolves (format drift), avoiding a wall of #idx.
• The grouping card renders label + resolved paths with a collapsible raw response for audit.

No on-disk format changes; existing sessions render retroactively.

Scope notes

• No streaming/truncation-salvage. Indices already shrink the output ~10x; a truncation that still happens falls back to per-file dispatch, the same as the path version did. Kept intentionally out of scope.
• No lenient int decoding. Prompts require integer indices; an occasional format drift falls back to per-file (existing behavior). Left as a possible future hardening if real usage shows it is needed.

Testing

• go build ./... and go vet ./... pass.
• internal/agent and internal/viewer test suites pass, covering index parsing, out-of-range/duplicate handling, index→path resolution, format-drift fallback, and end-to-end viewer rendering.

[github-actions]

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