git: add task delivery results, commit history, and merge readiness

[guicybercode]

Context

Status and diff show what changed, but a task-oriented product also needs to explain what an agent team delivered relative to the exact base revision.

Goal

Add a Task Result view that combines Git history, commits, changed files, bounded diffs, test evidence, and a merge-readiness summary.

Scope

• Record the task base OID and current head OID.
• Show commits created by the task branch and per-file diffs.
• Attach structured test/check results and agent summaries with provenance.
• Detect divergence from the base branch and refresh explicitly.
• Allow the user to mark a task ready for review or delivered.
• Export a Markdown result summary suitable for a pull request description.
• Link findings back to sessions, roles, and shared notes.

Acceptance criteria

• All comparisons use exact OIDs, not ambiguous branch names.
• Binary and oversized diffs have explicit bounded representations.
• A moved/deleted branch or rewritten history produces a recoverable state.
• Test evidence distinguishes observed command results from agent claims.
• No operation automatically merges, rebases, resets, commits, or pushes.
• Dirty worktree changes remain visible and cannot be mistaken for committed output.
• Export contains no env values, terminal replay, or hidden secrets.
• Real-Git tests cover multiple commits, rename, binary files, divergence, and detached HEAD.

Out of scope

Hosting-provider PR creation and automatic merge.