fix(web): preserve source revisions in chat citation links (#1205)

* fix(web): preserve source revisions in chat citation links

`convertLLMOutputToPortableMarkdown` was not passing `revisionName` to
`getBrowsePath`, so every copied citation link silently resolved to the
repo's default branch — even when the source was attached at a non-HEAD
revision. Plumb file sources through both callsites so the conversion
can resolve each reference to its source and use that source's revision.

Also tighten `get_diff` source emission: one source per path at head for
added/modified files, base for deletes, both sides for renames. The old
behavior emitted a duplicate unreachable source at base for every
modified file that the reference resolver would have picked first.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* docs: add CHANGELOG entry for #1205

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>