Simplify recurring agent test and commit paths

[hamzamerzic]

Problem

Common agent work still falls into two expensive failure paths: the default “fast” test command runs almost the complete backend suite and depends on Docker/frontend completeness, while /data commits sweep unrelated concurrent work. Reflection’s broad pre-run snapshot can itself capture another task’s edits.

Fix

• Make --fast a small hermetic host contract suite and keep full Docker execution as the explicit closeout/CI gate.
• Verify only the Node modules backend tests actually execute and isolate build/serving sentinels from the live runtime.
• Replace sweeping commits with a starting-revision, exact-path contract that preserves unrelated staged and unstaged work and stops on same-path concurrency.
• Remove Reflection’s broad safety snapshot and update its seeded guidance to use the same path-owned primitive.

Visual capture readiness remains in #321 and is intentionally not duplicated here.

Verification

• Fast contract suite: 57 passed in the clean review worktree.
• Commit helper tests cover unrelated staged/unstaged work, same-path concurrency, missing ownership arguments, and rejection of broad mode.
• Shell/Python syntax and compile checks passed.