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>

README.md

@@ -93,7 +93,7 @@ AMC requires Claude Code hooks to report session state. Add this to your `~/.cla
 | `bin/amc` | Launcher script — start/stop/status commands |
 | `bin/amc-server` | Python HTTP server serving the API and dashboard |
 | `bin/amc-hook` | Hook script called by Claude Code to write session state |
-| `dashboard-preact.html` | Single-file Preact dashboard |
+| `dashboard/` | Modular Preact dashboard (index.html, components/, lib/, utils/) |
 
 ### Data Storage
 
@@ -134,8 +134,10 @@ The `/api/respond/{id}` endpoint injects text into a session's Zellij pane. Requ
 - `optionCount` — Number of options in the current question (used for freeform)
 
 Response injection works via:
-1. **Zellij plugin** (`~/.config/zellij/plugins/zellij-send-keys.wasm`) — Preferred, no focus change
-2. **write-chars fallback** — Uses `zellij action write-chars`, changes focus
+1. **Zellij plugin** (`~/.config/zellij/plugins/zellij-send-keys.wasm`) — Required for pane-targeted sends and Enter submission
+2. **Optional unsafe fallback** (`AMC_ALLOW_UNSAFE_WRITE_CHARS_FALLBACK=1`) — Uses focused-pane `write-chars` only when explicitly enabled
+
+AMC resolves the Zellij binary from PATH plus common Homebrew locations (`/opt/homebrew/bin/zellij`, `/usr/local/bin/zellij`) so response injection still works when started via `launchctl`.
 
 ## Session Statuses
 
@@ -159,9 +161,17 @@ Response injection works via:
 - Zellij (for response injection)
 - Claude Code with hooks support
 
-## Optional: Zellij Plugin
+## Testing
 
-For seamless response injection without focus changes, install the `zellij-send-keys` plugin:
+Run the server test suite:
+
+```bash
+python3 -m unittest discover -s tests -v
+```
+
+## Zellij Plugin
+
+For pane-targeted response injection (including reliable Enter submission), install the `zellij-send-keys` plugin:
 
 ```bash
 # Build and install the plugin