fix(review-diff): discard a fully-staged file back to HEAD (#2318)

[sinelaw]

Summary

Fixes #2318.

In Review Diff, the file-level Discard action (D) ran git checkout -- <path>, which only rewrites the working tree from the index. When a file's change is entirely staged (clean working tree), that command is a no-op — yet the status bar still reported Discarded: <file>, telling the user an irreversible discard succeeded while the staged change remained in the index, on disk, and in the diff.

Fix

• Reset the file all the way back to its committed state with git checkout HEAD -- <path>, which restores both the index entry and the working tree to HEAD, so staged and unstaged edits alike are dropped (the obvious meaning of the "this cannot be undone" discard dialog).
• Honor the command's exit code: only report Discarded when it actually succeeded; otherwise surface Discard failed: … instead of a false success.

The untracked-file branch (rm) is unchanged.

Testing

• New e2e test test_issue2318_discard_fully_staged_file_reverts_to_head: stages a change so the working tree is clean, opens Review Diff, discards the file via D, and asserts the change disappears from the diff and that neither the working tree nor the index retains it. The test times out against the old git checkout -- <path> path (the change is never discarded) and passes with the fix.
• All 9 existing discard-related e2e tests pass, including the pre-existing hunk-discard regression test (Review Diff: "Discard hunk" fails with "patch does not apply" even when patch is valid #2117) — no regression.
• Plugin type-check (tsc --strict) passes.

Manual validation

Built the fresh binary and reproduced the issue's exact scenario in tmux (commit a file, stage a one-line change, Review Diff → D → confirm):

• Before: staged change survived, status falsely said Discarded.
• After: UI shows "No changes to review", status shows a truthful Discarded: calc.py, git status --short is empty, git diff --cached --quiet returns 0, and calc.py on disk is back to its committed contents.

https://claude.ai/code/session_01FqagsaYk1AERmbNY5GbnK2

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