fix(zellij): robust binary resolution and two-step Enter injection

Two reliability fixes for response injection:

1. **Zellij binary resolution** (context.py, state.py, control.py)
   
   When AMC is started via macOS launchctl, PATH is minimal and may not
   include Homebrew's bin directory. The new `_resolve_zellij_bin()`
   function tries `shutil.which("zellij")` first, then falls back to
   common installation paths:
   - /opt/homebrew/bin/zellij (Apple Silicon Homebrew)
   - /usr/local/bin/zellij (Intel Homebrew)
   - /usr/bin/zellij
   
   All subprocess calls now use ZELLIJ_BIN instead of hardcoded "zellij".

2. **Two-step Enter injection** (control.py)
   
   Previously, text and Enter were sent together, causing race conditions
   where Claude Code would receive only the Enter key (blank submit).
   Now uses `_inject_text_then_enter()`:
   - Send text (without Enter)
   - Wait for configurable delay (default 200ms)
   - Send Enter separately
   
   Delay is configurable via AMC_SUBMIT_ENTER_DELAY_MS env var (0-2000ms).

3. **Documentation updates** (README.md)
   
   - Update file table: dashboard-preact.html → dashboard/
   - Clarify plugin is required (not optional) for pane-targeted injection
   - Document AMC_ALLOW_UNSAFE_WRITE_CHARS_FALLBACK env var
   - Note about Zellij resolution for launchctl compatibility

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

amc_server/context.py

@@ -1,3 +1,4 @@
+import shutil
 from pathlib import Path
 import threading
 
@@ -10,6 +11,27 @@ CODEX_SESSIONS_DIR = Path.home() / ".codex" / "sessions"
 # Plugin path for zellij-send-keys
 ZELLIJ_PLUGIN = Path.home() / ".config" / "zellij" / "plugins" / "zellij-send-keys.wasm"
 
+
+def _resolve_zellij_bin():
+    """Resolve zellij binary even when PATH is minimal (eg launchctl)."""
+    from_path = shutil.which("zellij")
+    if from_path:
+        return from_path
+
+    common_paths = (
+        "/opt/homebrew/bin/zellij",  # Apple Silicon Homebrew
+        "/usr/local/bin/zellij",     # Intel Homebrew
+        "/usr/bin/zellij",
+    )
+    for candidate in common_paths:
+        p = Path(candidate)
+        if p.exists() and p.is_file():
+            return str(p)
+    return "zellij"  # Fallback for explicit error reporting by subprocess
+
+
+ZELLIJ_BIN = _resolve_zellij_bin()
+
 # Runtime data lives in XDG data dir
 DATA_DIR = Path.home() / ".local" / "share" / "amc"
 SESSIONS_DIR = DATA_DIR / "sessions"