chore: drop the Codex prompt-file workaround once Codex exposes MCP prompts

[ericfitz]

Summary

agentbus init --global writes ~/.codex/prompts/agentbus.md because Codex does not yet surface MCP server prompts, so Codex users cannot invoke the server's init prompt the way Claude Code users invoke /agentbus:init. Once Codex supports MCP prompts (upstream: openai/codex#5059, still open as of 2026-09-08), the custom prompt file is redundant and the special case should be removed.

Acceptance Criteria

• Feature request: MCP prompts support openai/codex#5059 is closed and a released Codex version exposes MCP prompts.
• agentbus init --global no longer writes ~/.codex/prompts/agentbus.md (internal/cli/init.go, codexPrompt).
• The corresponding assertion in internal/cli/init_test.go is removed or inverted.
• docs/install.md no longer mentions the Codex prompt file and documents the MCP prompt path for Codex instead.
• Consider cleaning up an existing ~/.codex/prompts/agentbus.md on the next init --global, or document that users may delete it.

Notes

Deferred from the v2 implementation; see HANDOFF.md/PROGRESS.md entries dated 2026-09-08. Blocked on the upstream Codex change; nothing to do until it ships.