feat(session): per-session git identity from session name

[DerekfromMadison]

What

Each CodeMan session now commits under its own git identity, derived from the session's display name: a session named w1-axon authors commits as w1-axon <w1-axon@codeman.local>.

Why

Running several sessions in parallel, every commit was attributed to the same identity — whatever the repo's local user.name happened to be. Per-session identity makes git log attribution and per-session rollback clean when multiple agents touch the same repo.

How

• New setGitIdentityEnvVars(muxName, name) in tmux-manager.ts, mirroring the existing setOpenCodeEnvVars (uses tmux setenv, inherited by panes, invisible in ps).
• Called in createSession (just before respawn-pane) and in updateSessionName.
• Sets GIT_AUTHOR_* and GIT_COMMITTER_* — env vars, because they're the only thing that overrides a repo-local user.name, and they work even when multiple sessions share one working directory (per-directory git config can't tell them apart).

Behavior

• The tmux session env persists across respawns, so a rename re-stamps the identity and the next (re)spawn — resume / clear / pane respawn — commits under the new name.
• A currently-running process keeps its spawn-time identity until it respawns — by design.
• Email is a stable slug: lowercased, runs of non-[a-z0-9._-] collapse to -, e.g. auth analysis becomes auth-analysis@codeman.local.

Verified live

9 concurrent sessions each commit under their own name; a real commit from a session named id is authored id <id@codeman.local>, overriding the repo-local identity. One session caught mid-rename showed its session env updated while the still-running process kept its spawn-time name — exactly as documented above.