feat: Set and Forget - Seamless Gas Town Integration (#255)

Adds shell integration for automatic Gas Town context detection.

Features:
- `gt enable` / `gt disable` - Global on/off switch
- `gt shell install|remove|status` - Shell integration management
- `gt rig quick-add [path]` - One-command project setup
- `gt uninstall` - Clean removal with options
- Shell hook auto-sets GT_TOWN_ROOT/GT_RIG on cd

Implementation:
- XDG-compliant state storage (~/.local/state/gastown/)
- Safe RC file manipulation with block markers
- Environment overrides (GASTOWN_DISABLED/ENABLED)
- Doctor check for global state validation

Co-authored-by: Sohail Mohammad <sohailm25@gmail.com>

internal/wrappers/scripts/gt-opencode

@@ -0,0 +1,18 @@
+#!/bin/bash
+# ABOUTME: Wrapper script that runs gt prime before launching opencode.
+# ABOUTME: Ensures Gas Town context is available in OpenCode sessions.
+
+set -e
+
+gastown_enabled() {
+    [[ -n "$GASTOWN_DISABLED" ]] && return 1
+    [[ -n "$GASTOWN_ENABLED" ]] && return 0
+    local state_file="$HOME/.local/state/gastown/state.json"
+    [[ -f "$state_file" ]] && grep -q '"enabled":\s*true' "$state_file" 2>/dev/null
+}
+
+if gastown_enabled && command -v gt &>/dev/null; then
+    gt prime 2>/dev/null || true
+fi
+
+exec opencode "$@"