[codex] docs: add Codex install note

[ketpatil77]

Description

README lacked a practical note for Codex and other MCP clients that cannot reliably inherit a shell PATH. That makes command = "headroom" brittle for uv-installed setups.

Changes Made

• Added a short Codex/global-install section to README.
• Documented uv tool install "headroom-ai[all]" and command -v headroom.
• Showed the absolute-path MCP config pattern.

Testing

• Reviewed the README diff for placement and command correctness.
• Confirmed the example path flow matches the documented uv install pattern.

Impact

Users can install Headroom once as a persistent uv tool and point Codex/MCP at a stable binary path instead of depending on ambient PATH inheritance.

[github-actions]

PR governance

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