feat(harness): add /handoff-session command

[rdemeritt]

What

Adds a new /handoff-session slash command that snapshots the current session into a dense, paste-ready handoff prompt.

Why

/compact is slow and can carry stale or hallucinated context forward. /handoff-session produces a clean, fact-grounded prompt you paste into a fresh session to reset context with zero drift.

How it works

1. Probe ground truth — reads the active client lock + git branch/status/log (repo-relative) so the handoff is anchored in real state, not memory.
2. Synthesize the session — task & goal, critical files (cross-checked against git status), ordered next steps, and decisions/constraints the next session must not re-litigate.
3. Build a fixed-structure prompt — consistent sections every time.
4. Copy + print — pipes straight to the clipboard via heredoc (cross-platform: pbcopy / xclip / wl-copy / clip.exe) and prints a fenced backup. Writes no files to disk.

Notes

• Accepts an optional focus arg, e.g. /handoff-session focus on the CI fix.
• Read/probe only — does not modify any files.
• Genericized from the internal version: repo-relative git probes and cross-platform clipboard handling.