Fix master agent launch race on cold tmux server

[dpvlv]

Fixes #67.

Problem

The first agent in a pack — always the master (e.g. specifier) that reads the board card and drives handoffs — intermittently never launches. Its pane is left at the shell prompt with the launch command typed but not executed (sometimes mangled into a broken continuation line), so no card is read and the whole pack idles waiting for a handoff that never comes.

Root cause

launch-role! delivers the command with a single tmux send-keys <command> "Enter". The master's pane is the first shell on a cold tmux server; with an async prompt (starship/powerlevel10k) the prompt is still initializing when the keys arrive, so the trailing Enter (and occasionally a character) is dropped during a prompt redraw. Later roles start on a warm server and succeed, so only the master fails. Because it depends on cold-server prompt timing, no fixed SWARMFORGE_AGENT_START_DELAY_MS is reliable — and that delay is currently skipped for index 0 ((when (pos? index) ...)) anyway.

Change

• launch-role! now sends the command text and its Enter as separate keystrokes (after C-c/C-u to clear any partial line), then verifies the agent actually took over the pane via pane_current_command, re-sending until it does, up to a retry cap. This is adaptive to any prompt speed and self-heals a swallowed launch instead of leaving the master dead.
• launch-roles! now delays before every role (including index 0) so the cold master shell gets the same head start as the others.
• New helpers: shell-pane-commands, pane-current-command, send-role-launch-keys!.
• New env knobs: SWARMFORGE_AGENT_LAUNCH_RETRIES (default 8), SWARMFORGE_AGENT_ENTER_DELAY_MS (default 400).

Verification

On macOS/zsh with starship, a cold ./swarm + Open Project now launches all four roles with pane_current_command = <agent> (no zsh left behind). The master (specifier) reads the board card, dry-checks it, and hands off to coder (card moves specifier → coder), which begins working — the full pipeline flows on the first try. File parses under bb and the built-in --test-agent-start-delay entrypoint still works.

No behavior change for roles that already launched correctly; the verify-and-retry succeeds on the first attempt for them.