feat(arc): auto-materialize Arc pi-subagents

[bfirestone]

Summary

This PR moves forward the Arc pi-subagents coordination work without the Taskplane spike changes. It keeps the small Codex nano model fix and focuses on making Arc specialists available automatically, easier to monitor, and safer to refresh when model/profile configuration changes.

What changed

Auto-materialized Arc specialists

• Adds extensions/arc/subagents.ts as the dedicated materialization module for Arc specialist agent definitions.
• Materializes bundled Arc specialists into the pi-subagents user-scope agent directory by default so users do not have to run /arc-subagents-sync as a normal setup step.
• Preserves non-generated user files and only manages files marked as generated by @sentiolabs/pi-arc.
• Records generation metadata in rendered agent markdown, including source path/version/hash information, so stale or shadowed specialist definitions are easier to diagnose.
• Quotes colon-bearing YAML frontmatter descriptions correctly and adds regression coverage for the rendered markdown shape.

Safer fallback and shadow handling

• Keeps /arc-subagents-sync as a deprecated repair/backcompat command instead of the primary path.
• Reports project-scope and user-scope shadows so users can tell when legacy generated files may override or obscure the auto-materialized specialists.
• Constrains legacy fallback behavior to directory-level failures instead of falling back on individual file write failures, making partial failures visible rather than silently writing to an older path.
• Adds coverage for modern user directory preference, legacy fallback behavior, project shadow reporting, and per-file target failures.

Model profile coordination

• Refreshes generated Arc subagents after /arc-models saves profile changes, so generated frontmatter stays aligned with the configured Arc model profiles.
• Keeps the Codex nano-tier fix by replacing the unavailable openai-codex/gpt-5.4-nano default with openai-codex/gpt-5.4-mini.
• Updates tests and docs so model profile defaults and generated specialist metadata stay in sync.

Arc skill and docs guidance

• Updates README and Arc skills to document that Arc specialists are auto-materialized and that /arc-subagents-sync is now a repair/backcompat path.
• Clarifies /subagents-status semantics: it is for active/recent async runs, while installed/idle specialists are discovered via agent listing.
• Adds optional intercom escalation instructions to Arc specialist prompts without bundling pi-intercom as a package dependency.
• Updates build/plan/review guidance to prefer async pi-subagents Arc specialist dispatch when available, while preserving the self-contained arc_agent fallback when specialists cannot load.

Explicitly not included

• The Taskplane adapter/exporter spike is intentionally excluded from this branch so pi-subagents improvements can proceed independently of current Taskplane package conflicts.

Validation

• npm test — 55 tests passed