Add hermod mvp

[zerlok]

Hermod MVP: one-command bridge to a remote dev box

Establishes the Hermod MVP — a single command that mirrors your working directory to a
remote sandbox, drops you into a persistent session running there, and cleans up on the
way out. The repo is spec-driven: behavior was defined as OpenSpec specs before any code
was written, and this PR ships both the specs and their implementation.

Why

Working on a remote dev box means hand-wiring three tools every session — Mutagen to mirror
files, SSH to get a shell, tmux to survive drops — plus threading git identity through so
commits are attributed correctly, and tearing it all down cleanly on the way out. The
friction is high enough that people avoid remote sandboxes even when the sandbox is where
the work belongs. Hermod collapses that setup into one command.

What Changes

The core loop is sync → attach → pause/teardown:

0. you run `hermod prod-box` from your project directory
1. Mutagen session starts   →  working dir mirrors to the sandbox, live
2. SSH into the sandbox      →  attach to (or create) a persistent tmux session
3. git identity carried in   →  GIT_AUTHOR_NAME / GIT_AUTHOR_EMAIL exported, commits are yours
4. you work in tmux          →  the agent (or shell) runs remotely; edits sync back
5. you detach                →  session alive → sync PAUSES (resume later);
                              session gone  → sync TEARS DOWN cleanly

• cli — parse one invocation into a resolved, immutable intent: sandbox host alias,
  session name (-s), working directory (-C), dry-run (-n), -- passthrough to the
  remote command, ssh-config path, log level.
• file-sync — the Mutagen mirror lifecycle: open (create or resume), flush before
  work, pause, and terminate.
• remote-session — the remote interactive session (a tmux window reached over SSH):
  attach-or-create a persistent window, run a remote command, probe liveness.
• git-identity — read local user.name/user.email and carry them into the remote
  session's environment; a soft feature that never hard-fails when identity is unset.
• session-orchestration — the end-to-end flow and its central decision: remote-session
  liveness after detach is the single source of truth for pause vs teardown.
  Guarantees mirror teardown even on interrupt/error.

Dry-run (-n) prints a copy-pasteable plan of side effects; read-only probes (git identity,
sync status) stay real even under --dry-run.

Also included: Makefile (build/test/lint + dev workflow), CI and release GitHub Actions
workflows, and bash completions installed via make install.

Implementation notes

• Greenfield Go module — a single hermod binary with no runtime dependencies. It
  orchestrates tools already on the machine (Mutagen, ssh, tmux) and reads
  ~/.ssh/config; it does not bundle or provision them.
• Domain packages: internal/cli, internal/mirror, internal/sandbox, internal/git,
  internal/shell (local/ssh/tmux/login-shell execution, quoting, exit codes).
• Archives the completed add-hermod-mvp OpenSpec change and syncs its 5 delta specs into
  openspec/specs/ as the project's baseline.