bug: pi-coding-agent slash-command prompts missing arg placeholders

[taqtiqa-mark]

Problem

pi-coding-agent users use .pi/prompts/opsx-*.md for /opsx-propose <desc> → OpenSpec change+artifacts.

OpenSpec prompts lack $@ placeholders → /opsx-propose <text> discards <text> (pi expands verbatim).

Root cause analysis:

Root cause:

/opsx-propose is a Pi prompt template (https://www.npmjs.com/package/@mariozechner/pi-coding-agent?activeTab=docs#prompt-templates) from .pi/prompts/opsx-propose.md.
Typing /opsx-propose <text> in Pi's editor triggers expansion: Pi replaces the command+args with the verbatim template Markdown (instructions/steps), discarding <text> because the template lacks arg placeholders ($@, ${@}, etc.).

• Pi template assumes "The argument after /opsx:propose is..." but cannot interpolate it.
• Result: Editor fills with fixed instructions; agent gets no <text>, so per step 1, it AskUserQuestion for description (ignores passed text).
• Note: Template uses /opsx:propose (colon) for display/docs, but pi command is /opsx-propose (no colon, matches filename).

Evidence:

• .pi/prompts/opsx-propose.md: No $@.
• Pi docs (https://www.npmjs.com/package/@mariozechner/pi-coding-agent?activeTab=docs#prompt-templates): Args
  only inserted if placeholders used; else discarded on expand.
• Pi example (https://github.com/badlogic/pi-mono/blob/d9cfa115bed37c6036734ec3ab5ad1cbbda6f36a/.pi/prompts/pr.md?plain=1#L4)
• Skills are separate (/skill:openspec-propose appends User: to SKILL.md).

Describe a Workaround

Edit .pi/prompts/opsx-propose.md to use $@:

   **Input**: The argument after `/opsx:propose` is "$@". Use this as the change name (kebab-case) OR ...

Then /opsx-propose foo expands with "foo" inserted.

Fix works (insert $@ + /reload), but manual/fragile—dupes schema (artifacts/status/instructions).

Describe a Solution

CLI gen pi prompts w/ dynamic placeholders:

openspec gen pi-prompt propose > .pi/prompts/opsx-propose.md
# Outputs:
**Input**: Args: **$@**. Derive name/desc from "$@".
1. openspec new change "`kebab($@)`"
2. Loop `openspec status --json`...
[Schema-driven steps/deps/instructions]

• --format pi-slash: $@ interp.
• --workflow {apply,continue,...}
• --out .pi/prompts/

Benefits:

• Schema changes → auto-updates.
• No maint dup.
• npm pi packages share.

Alternative Solutions:

• Static prompt files: No arg sub.
• Custom pi ext: Reinvents schema parse.

Additional Details

• Tested v1.2.0: Full flow works post-fix.
• Ex: /opsx-propose "add container scripts ..." → chore-add-containerized-test-lint-scripts (4 artifacts).
• pi docs: https://pi.dev/docs/prompt-templates.md

[TabishB]

Should be fixed as part of: #950