flow init: install SKILL.md to Codex / Cursor / Gemini skill dirs, not only ~/.claude/

[vishnukv-facets]

Summary

Today flow init and flow skill install write SKILL.md only to ~/.claude/skills/flow/. The same SKILL.md format works as-is for Codex (~/.codex/skills/), Cursor (~/.cursor/skills/), and Gemini CLI (~/.gemini/skills/) — all of which already expose a Claude-Code-compatible skills loader. Filling those dirs at install time is the minimum change required for those agents to discover and load the flow skill.

Background — the gap on main

• internal/app/skill.go::skillInstallPath() hardcodes ~/.claude/skills/flow/SKILL.md and is the only target everywhere (flow init, skill install, skill update, skill uninstall, maybeAutoUpgradeSkill).
• The embedded SKILL.md on main makes no claims about other agents.
• The unmerged facets/codex-support branch adds full flow do --agent codex runtime support but does NOT change the install layer.
• Result: even with the Codex branch merged, ~/.codex/skills/flow/ would still be empty unless the user copies SKILL.md by hand. Multi-agent install is missing for everyone.

Proposed behavior

skill.go exposes a skillTargets() function returning one entry per agent whose ~/.<agent>/ parent dir exists on disk:

Agent	Parent dir	Skill path
Claude Code	~/.claude/	~/.claude/skills/flow/SKILL.md
Codex CLI	~/.codex/	~/.codex/skills/flow/SKILL.md
Cursor	~/.cursor/	~/.cursor/skills/flow/SKILL.md
Gemini CLI	~/.gemini/	~/.gemini/skills/flow/SKILL.md

flow init, flow skill install, flow skill update, flow skill uninstall, and maybeAutoUpgradeSkill all loop over the discovered targets and operate on each — idempotently, with a per-agent VERSION sidecar so each agent can be upgraded independently.

Auto-discover only. If ~/.codex/ does not exist, the Codex target is silently skipped (the user does not have Codex installed). We do NOT eagerly create ~/.codex/ ourselves — that would litter the home dir for users who do not use that agent.

Hook installation is unchanged. The SessionStart hook in ~/.claude/settings.json is Claude-specific and stays Claude-only. Codex/Cursor/Gemini have their own hook mechanisms (or none) — out of scope here.

Why this is the right shape

• Smallest possible change. No new commands, no new flags, no flow do changes, no DB schema work. Just a target list.
• Future-proof. Adding a new agent (e.g. opencode if it grows a standard skills dir) is a single entry in skillTargets().
• Reversible. flow skill uninstall cleans up across all agents the same way it cleans up Claude today.
• Compatible with the Codex branch. When facets/codex-support is rebased and merged, the Codex install path will already be in place — the runtime work and the install work compose cleanly.

Out of scope (separate issues)

• flow do --agent codex|opencode|... runtime dispatch (already partially built on facets/codex-support — separate review).
• session_provider DB column.
• Agent-specific hook installation beyond Claude's SessionStart.
• Opencode support — its layout (~/.opencode/ = node bin/) doesn't match the global-skills-dir pattern. Revisit if opencode adopts one.

Acceptance

• flow init on a machine with all four agent dirs writes SKILL.md into all four skills/flow/ paths.
• flow init on a machine with only ~/.claude/ writes only to Claude — no spurious dir creation.
• flow skill install, update, uninstall operate over the full discovered set, idempotent on re-run.
• maybeAutoUpgradeSkill refreshes every present agent's SKILL.md when the binary version changes.
• Per-agent VERSION sidecars (~/.<agent>/skills/flow/VERSION) track upgrades independently.
• Claude's SessionStart hook installation is unchanged.
• New test in skill_test.go covers the multi-target case via $HOME override.
• make test passes.