feat(background-shell): add persistent tmux session plugin

Adds a background-shell plugin providing the `bgsh` helper: named tmux
sessions that survive across tool calls and SSH disconnects, for work
ordinary tool calls cannot hold — interactive prompts, REPLs, TUIs, and
jobs that must outlive a connection.

The skill description is written as a hard gate: over-triggering is the
primary failure mode, so it redirects to plain Bash for everyday work.

- tty is the default mode (output stays in the pane, cd persists);
  --log is the alternate for one-shot captured payloads
- a local registry keeps remote sessions findable, storing *where to
  look* and never *what is running* — every read verifies against the
  host, and prune refuses to act on an unreachable one (UNSURE != STALE)
- bin/bgsh is on the plugin PATH, so no symlink step is needed
- SKILL.md carries a "why bgsh does it this way" table: each row is a
  real failure whose output was indistinguishable from success

Passes `claude plugin validate` with no warnings; bgsh is shellcheck
clean and bash 3.2 compatible.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

background-shell/.claude-plugin/plugin.json

@@ -0,0 +1,9 @@
+{
+  "name": "background-shell",
+  "version": "1.0.0",
+  "description": "Persistent named tmux sessions for work that ordinary tool calls cannot hold: interactive prompts, REPLs, TUIs, and jobs that must outlive an SSH connection. Ships the bgsh helper plus a registry that keeps remote sessions findable from the machine that started them.",
+  "author": {
+    "name": "Adam Knight"
+  },
+  "keywords": ["tmux", "shell", "ssh", "session", "remote", "interactive", "tty"]
+}