fix(session): return proper exit codes for cmd_continue and fork_agent

- fork_agent() now returns exit code instead of echoing status
- cmd_continue() returns exit code 2 when max agents reached
- cmd_continue() returns exit code 1 for general errors (instead of exit 1)
- ensure_worktree() returns exit code 2 for max agents condition
- Add EXITCODES.md documenting all exit codes

Closes #248

EXITCODES.md

@@ -0,0 +1,61 @@
+# Exit Codes
+
+This document describes the exit codes used by kugetsu commands.
+
+## Exit Codes
+
+| Exit Code | Meaning |
+|-----------|---------|
+| 0 | Success - operation completed successfully |
+| 1 | General error - worktree/session/validation failed |
+| 2 | Max concurrent agents reached (MAX_CONCURRENT_AGENTS limit) |
+
+## Commands
+
+### cmd_continue / cmd_start
+
+The `cmd_continue` command (aliased to `cmd_start`) returns exit codes to indicate the result of the operation:
+
+- **Exit 0**: Success - agent forked successfully
+- **Exit 1**: General error - worktree/session/validation failed
+- **Exit 2**: Max concurrent agents reached
+
+### Internal Functions
+
+#### fork_agent()
+
+Forks a new agent session for a given worktree.
+
+- **Return 0**: Success - agent forked successfully
+- **Return 1**: General error - invalid worktree path or other failure
+
+#### ensure_worktree()
+
+Ensures a worktree exists for the given issue reference.
+
+- **Return 0**: Success - worktree existed or was created
+- **Return 1**: General error - base session not found or worktree creation failed
+- **Return 2**: Max concurrent agents reached
+
+## Daemon Integration
+
+These exit codes are designed to help the queue daemon distinguish between recoverable and non-recoverable errors:
+
+- **Exit 2 (max agents)**: This is a recoverable error - the daemon can retry later when agents become available
+- **Exit 1 (general error)**: Non-recoverable - the task should be marked as failed
+
+## MAX_CONCURRENT_AGENTS
+
+The maximum number of concurrent development agents is controlled by the `MAX_CONCURRENT_AGENTS` configuration variable (default: 3).
+
+Set this in your `config` file:
+
+```bash
+MAX_CONCURRENT_AGENTS=5
+```
+
+Or via environment variable:
+
+```bash
+export MAX_CONCURRENT_AGENTS=5
+```