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/mixins/state.py

@@ -9,6 +9,7 @@ from amc_server.context import (
     SESSIONS_DIR,
     STALE_EVENT_AGE,
     STALE_STARTING_AGE,
+    ZELLIJ_BIN,
     _state_lock,
     _zellij_cache,
 )
@@ -143,7 +144,7 @@ class StateMixin:
 
         try:
             result = subprocess.run(
-                ["zellij", "list-sessions", "--no-formatting"],
+                [ZELLIJ_BIN, "list-sessions", "--no-formatting"],
                 capture_output=True,
                 text=True,
                 timeout=2,