feat: add MAESTRO_SESSION_RESUMED env var for hook control

Add environment variable that allows agent hooks to differentiate
between new sessions and resumed sessions. Since Maestro spawns a
new process per message (batch mode), session start hooks fire on
every turn. This env var lets users conditionally skip hooks.

- Set MAESTRO_SESSION_RESUMED=1 when --resume or --session flags present
- Document in README with example hook guard
- Add note in Agent Provider UI linking to documentation

Fixes #42

Co-authored-by: TQ White II (@tqwhite)

README.md

@@ -437,6 +437,26 @@ Each task executes in a completely fresh AI session with its own unique session
 
 This isolation is critical for playbooks with `Reset on Completion` documents that loop indefinitely. Without it, the AI might "remember" completing a task and skip re-execution on subsequent loops.
 
+### Environment Variables {#environment-variables}
+
+Maestro sets environment variables that your agent hooks can use to customize behavior:
+
+| Variable | Value | Description |
+|----------|-------|-------------|
+| `MAESTRO_SESSION_RESUMED` | `1` | Set when resuming an existing session (not set for new sessions) |
+
+**Example: Conditional Hook Execution**
+
+Since Maestro spawns a new agent process for each message (batch mode), agent "session start" hooks will run on every turn. Use `MAESTRO_SESSION_RESUMED` to skip hooks on resumed sessions:
+
+```bash
+# In your agent's session start hook
+[ "$MAESTRO_SESSION_RESUMED" = "1" ] && exit 0
+# ... rest of your hook logic for new sessions only
+```
+
+This works with any agent provider (Claude Code, Codex, OpenCode) since the environment variable is set by Maestro before spawning the agent process.
+
 ### History & Tracking
 
 Each completed task is logged to the History panel with:

src/main/process-manager.ts

@@ -289,6 +289,14 @@ export class ProcessManager extends EventEmitter {
           env.PATH = standardPaths;
         }
 
+        // Set MAESTRO_SESSION_RESUMED env var when resuming an existing session
+        // This allows user hooks to differentiate between new sessions and resumed ones
+        // See: https://github.com/pedramamini/Maestro/issues/42
+        const isResuming = finalArgs.includes('--resume') || finalArgs.includes('--session');
+        if (isResuming) {
+          env.MAESTRO_SESSION_RESUMED = '1';
+        }
+
         logger.debug('[ProcessManager] About to spawn child process', 'ProcessManager', {
           command,
           finalArgs,

src/renderer/components/NewInstanceModal.tsx

@@ -602,6 +602,21 @@ export function NewInstanceModal({ isOpen, onClose, onCreate, theme, existingSes
               </div>
             )}
 
+            {/* Hook behavior note */}
+            <p className="text-xs mt-2" style={{ color: theme.colors.textDim }}>
+              Agent hooks run per-message. Use{' '}
+              <a
+                href="https://github.com/pedramamini/Maestro#environment-variables"
+                target="_blank"
+                rel="noopener noreferrer"
+                className="underline hover:opacity-80"
+                style={{ color: theme.colors.accent }}
+              >
+                MAESTRO_SESSION_RESUMED
+              </a>
+              {' '}to skip on resumed sessions.
+            </p>
+
             {/* Debug Info Display */}
             {debugInfo && (
               <div