Proposal: deliver the ruleset to subagents and forks, ship it as a Claude Code output style, make always-on a plugin option

Opening this before the PR, as CONTRIBUTING asks for new hooks and behaviour changes.

Problems observed in daily use (Claude Code 2.1.260)

1. Subagents never see the rules. The SessionStart hook injects the ruleset into the main conversation only. Every Agent-tool subagent answers in default style. Measured on 14 days of my own transcripts: 26 subagent runs, 0 carried the ruleset.
2. Forked sessions skip the hook. Since Claude Code 2.1.214 a forked session (/fork, /branch, --fork-session) reports SessionStart source fork, not resume. The matcher startup|resume|clear|compact does not fire for it.
3. The hook is compactable and node-dependent. Its output lives in conversation context, so it is summarised away on compaction and re-sent uncached every turn. It also silently does nothing when node is not on PATH (launchd, bare tmux).
4. Always-on is a hidden flag file. Users have to know about ~/.claude/.i-have-adhd-always; there is no visible switch.

Proposed changes (all opt-in, no existing behaviour removed)

• Add a SubagentStart hook using the same launcher; it emits hookSpecificOutput.additionalContext. Skips agent_type: fork, which inherits the parent conversation.
• Add fork to the SessionStart matcher.
• Ship output-styles/i-have-adhd.md with the same body as SKILL.md (test-enforced equality, keep-coding-instructions: true). Claude Code resolves it as i-have-adhd:i-have-adhd. When a user selects it, the SessionStart hook stands down so the rules are not injected twice.
• Declare a userConfig option always_on (boolean, default false) in plugin.json; Claude Code exports it to the hook as CLAUDE_PLUGIN_OPTION_ALWAYS_ON. Add I_HAVE_ADHD_ALWAYS_ON=1 for Codex. The flag file keeps working. A new .i-have-adhd-off file wins over all of them.

Default behaviour after the change is identical to today: install does nothing until the user invokes the skill or turns always-on on.

Verification so far

• 60 unit tests across node, sh and PowerShell runtimes, including banner byte-equality across runtimes.
• Live probe on 2.1.260: with outputStyle: "i-have-adhd:i-have-adhd" the model quotes the ruleset from its system prompt; the bare name i-have-adhd does not resolve.

Happy to split further or drop any part. Working branch: https://github.com/ahma/i-have-adhd (branch opus5-ux). Drafted with Claude Code on behalf of @ahma, who reviews and submits.