Add optional agent-tool installs via skillflag discovery

[osolmaz]

Summary

Extend simpledoc install to optionally install the bundled skill into external agent tool stores (e.g. Codex, Claude, Cursor, etc.). This should be driven by a discovery mechanism in skillflag so SimpleDoc can list supported agent targets dynamically.

Motivation

Right now SimpleDoc only writes skills/simpledoc/SKILL.md inside the repo. Users often want the skill installed into their agent tool’s repo/user scope as well. We should make that a first-class option, but avoid hard-coding the list of agents in SimpleDoc.

Proposal

• Add a skillflag discovery API/CLI (e.g. npx skillflag agents --json or similar) that returns supported agent targets and scopes.
• Update simpledoc install to:
  • ask whether to install the skill into external agent stores
  • show the list from skillflag
  • run the appropriate npx skillflag install --agent <name> --scope <repo|user> commands
• Keep repo-local file install as today, and make external installs optional.

Acceptance criteria

• skillflag exposes a machine-readable list of agent targets and supported scopes.
• simpledoc install uses that list (no hard-coded agent list).
• External install remains opt-in and does not break non-interactive/CI flows.

Notes

• This is blocked on skillflag exposing the discovery list. If that’s not ready, gate the feature behind a check and skip with a clear message.