feat(web): unstage a partially staged file from the diff view

[MTVaught]

Problem

In the diff panel's All view, a partially staged file (staged changes plus newer edits on disk) only offered a Stage button. Getting its staged part back out of the index meant switching to the Staged view first.

Fix

A partially staged file now shows both buttons in the All view: plus stages the remaining edits, minus resets the file's index entry to HEAD. Fully staged files keep only Unstage and unstaged files keep only Stage. The filtered views are unchanged, since each already implies the direction.

Follow-up to #100.

Built with Claude Fable 5.1 via Claude Code in T3 Code.

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.5 KiB	13.6 KiB	+48 B (+0.3%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.1 KiB	7.1 KiB	−3 B (−0.0%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.5 KiB	6.5 KiB	+51 B (+0.8%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	56.2 KiB	57.0 KiB	+778 B (+1.4%)	66.4 KiB	✅
Codex	Live turn messages	9	8	−1 (−11.1%)	21	✅
Claude	Total thread wire	13.6 KiB	13.6 KiB	+26 B (+0.2%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.1 KiB	7.1 KiB	+10 B (+0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.5 KiB	6.5 KiB	+16 B (+0.2%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	57.8 KiB	57.8 KiB	+44 B (+0.1%)	66.4 KiB	✅
Claude	Live turn messages	8	9	+1 (+12.5%)	21	✅

Baseline: 47f898b · PR result: ddda28b · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 113.9 KiB
• Claude decoded thread snapshot: 114.6 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.