feat(coding-agent): let extensions expand skills in sendUserMessage

[andrebrait]

What

Honor expandPromptTemplates: true to allow Ponytail's pi extension to work on omp. For context: an omp-native extension was proposed to upstream Ponytail, but ultimately rejected because the pi extension works well enough. I did send a PR to fix how their pi extension interfaces with omp, but some things still require addressing on omp's side. This is one of them.

Related: DietrichGebert/ponytail#858 (the rejected omp-native extension) and DietrichGebert/ponytail#1050 (the Pi extension fix).

pi.sendUserMessage() now accepts expandPromptTemplates (default false), as Pi does. When it is set, a registered /skill:<name> command becomes the user-invoked skill message on every delivery path (idle, steer, follow-up, aside), and any other text gets prompt-template expansion. Extension, custom, and file slash commands still run only through prompt(), which handles an omitted deliverAs and aside while idle. If the extension passes attribution: "agent", the expanded skill body is sent as plain user-role text, the same as every other sendUserMessage path.

The shared #queueCustomMessage now drops a steer or follow-up, and promptCustomMessage resolves false, when an abort, a session change, or disposal lands during image preparation. Before, the message was queued after abort() had cleared the queues, which could restart the run the user had just stopped. The same race affected skill commands with images sent from RPC or the TUI while a turn was streaming.

Why

omp kept Pi's original hardcoded expandPromptTemplates: false and never added Pi's later opt-in. Pi extensions that register skill aliases therefore broke in omp. ponytail's /ponytail-review, for example, sent the literal /skill:ponytail-review text, and the model never received the skill.

Testing

• agent-session-send-user-message-expand.test.ts (9 tests) covers: default literal vs opt-in expansion, the disabled-skill-commands gate, an idle explicit follow-up staying queued, an expanded template that the submitted text can still remove, abort during the SKILL.md read, abort or disposal during image normalization (promptCustomMessage resolves false), agent attribution delivered as a user-role message, and a follow-up queued while streaming. The expansion, drop, and attribution tests failed before their fixes. Mutating the gate, queueOnly, queued template expansion, or rawText makes the matching branch test fail.
• 2633 related coding-agent tests pass (agent-session, rpc, acp, collab, extensions, task, input-controller, interactive-mode-loop). bun run check:ts passes. I didn't run check:rs because the PR has no Rust changes.
• I ran a real RPC session with the ponytail Pi extension and sent /ponytail-review focus on the diff. omp 18.6.2 sends the literal /skill: text to the model. This branch sends the skill body, ending in User: focus on the diff.
• Not covered: omp -p "/ponytail-help" still produces no turn. Print mode never waits for turns that an extension command starts with sendUserMessage, and this is the same for plain text. That needs a separate fix.

---

• bun check passes (only check:ts was run; see Testing)
• Tested locally
• CHANGELOG updated with the required attribution (if user-facing; internal issue fixes use issue links, external contributions add the PR link and contributor credit after creation)

[roboomp]

P2 — focused opt-in extension behavior; maintainer sign-off needed for the new sendUserMessage contract.
Should-fix: add external contributor/PR attribution to packages/coding-agent/CHANGELOG.md:11.
Verification blocked: focused bun test packages/coding-agent/test/agent-session-send-user-message-expand.test.ts fails at import because src/export/html/tool-views.generated.js is absent in this checkout; no assertions ran.
Thanks for the detailed repro and test coverage, @andrebrait.

[roboomp]

should-fix: This user-facing entry lacks the external-contribution attribution required by the changelog convention. Append ([#14608](https://github.com/can1357/oh-my-pi/pull/14608) by [@andrebrait](https://github.com/andrebrait)) so release notes credit the contributor and link the PR.

[andrebrait]

This is already in the reviewed commit. In 469f77c98e, CHANGELOG.md:11 ends with ([#14608](https://github.com/can1357/oh-my-pi/pull/14608) by [@andrebrait](https://github.com/andrebrait)). About the blocked verification: tool-views.generated.js is a build-generated file. With it present, bun test packages/coding-agent/test/agent-session-send-user-message-expand.test.ts passes 9/9 locally.