feat(spawn): [SI-16] enforce CODEX_FLEET_AGENT_NAME + propagate env

[NagyVikt]

Fail-fast guard plus explicit env propagation to the spawned codex CLI.

Summary

• Add fail-fast guard at the top of scripts/codex-fleet/claude-spawn.sh: if CODEX_FLEET_AGENT_NAME is unset, exit 2 with the documented FATAL message BEFORE any tmux operations or pane creation.
• Propagate the CODEX_FLEET_* family (AGENT_NAME, TIER, SPECIALTY, WORKER_CWD) explicitly through build_pane_cmd's env_str so the values survive codex CLI's env scrubbing and show up under printenv in the spawned worker.
• New smoke test scripts/codex-fleet/test/run-spawn-enforcement.sh covers:
  • Case 1: unset var -> exit 2 + FATAL banner.
  • Case 2: set var -> dry-run succeeds (exit 0, dry-run banner).
  • Case 2b: rendered env_str contains the CODEX_FLEET_* family.
  • Case 2c: mock-wrapper environ contains the CODEX_FLEET_* family after the rendered env(1) prefix runs.

Context

During the 2026-05-18 pt2 trading-edge run the supervisor observed worker panes self-identifying as the generic codex agent because CODEX_FLEET_AGENT_NAME was missing in the spawn env. Colony's matchmaker then merged multiple panes into one logical agent, causing overlapping claims and confused handoffs. The fail-fast guard makes this misconfiguration impossible to ignore, and the explicit CODEX_FLEET_* propagation closes the gap where the staged values were not visible to printenv inside the worker.

Test plan

• bash scripts/codex-fleet/test/run-spawn-enforcement.sh -> 13 pass, 0 fail.
• bash -n scripts/codex-fleet/claude-spawn.sh clean.
• Manual: bash scripts/codex-fleet/claude-spawn.sh --dry-run -n 1 with var unset -> exit 2 + FATAL banner.
• Manual: CODEX_FLEET_AGENT_NAME=test-fixture bash scripts/codex-fleet/claude-spawn.sh --dry-run -n 1 -> exit 0, dry-run banner.

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