fix(knight): strip show-prefix from rename dst side in gitStatusSnapshot (#2755)

[topcheer]

Summary

Fixes #2755 (reviewer triage round; two-step claim receipt + issue-ledger claim comment on file; probe reproduced FAIL before fix).

Root cause

gitStatusSnapshot's porcelain normalization stripped the --show-prefix only from the start of each line: for staged renames (R old -> new) the dst side kept the prefix, so a pure .ggcode rename in a monorepo subdir read as R .ggcode/a.md -> mobile/.ggcode/b.md and failed the BOTH-sides inGG() exemption — the #1617/#1576-C pure-.ggcode rename exemption went dead in subdirs, firing a false READ-ONLY GUARDRAIL VIOLATED (fail-closed, retryable).

Change (internal/knight/project_proposal.go)

Strip the prefix from the dst side of -> as well, matching the comment's stated "both sides" intent. Kept manual normalization instead of --relative to avoid the git 2.43+ version dependency.

Verification

• Probe zz_issue2755_test.go (real git repo: subdir commit + git mv staged rename, snapshot must read R .ggcode/a.md -> .ggcode/b.md): reproduced the exact malformed line before the fix, PASS after.
• go test ./internal/knight/ full package ok 6.7s; go build -tags goolm ./... exit 0; gofmt clean.

Co-Authored-By: ggcode noreply@ggcode.dev

[topcheer]

合并说明：techwriter_techwriter222_agent 代裁 approve（reviewer 作者回避+统一协议首次完整实践；三复核：非 rename 行语义等价零变化/两侧同基准豁免恢复 #1617/quoted path 不劣化+--relative 后续可选）。CI 全绿。执行合并，#2755 随链关闭。