Split ADR-process detail out of AGENTS.md #3988

Background

I had concerns about the ADR details in AGENTS.md - it seemed to be disproportionate detail for a top-level agent-instructions file.

Noticed issues

1. AGENTS.md states the status frontmatter convention applies to "Design and spec docs (anywhere under docs/)". That's broader than reality. When working on a PR, CodeRabbit read AGENTS.md's looser wording and incorrectly told Lisa to add status frontmatter to a plain developer guide (architecture.md). (see this PR [Docs] Architecture - point to AGENTS & CONTEXT.md as source of truth #3976) which is incorrect.
2. Claude noted: Also found: the design doc ADR-0000 points to for "the workflow in detail" (docs/superpowers/specs/2026-05-26-adr-workflow-design.md) was deleted after extraction (per the lifecycle it describes) — so AGENTS.md's inlined text and extract-adrs SKILLS.md are where this process exists in writing.

Plan to address

(A summary from a Claude Planning chat)

Goal: shrink AGENTS.md's ADR subsection to match its siblings' one-line-pointer style, move the procedural detail to a standalone doc, and fix the frontmatter-scope wording while we're at it.

• Create docs/agents/adr_process.md (comprehensive AI-instruction doc)
• Create docs/developer_guides/adr_process.md (hand-authoring steps only)
• Add nav entry to mkdocs.yml for developer_guides/adr_process.md
• Trim AGENTS.md ADR subsection to one-line-pointer style IMPORTANT
• Add discoverability pointer to docs/adr/index.md

I also intend to have some high level process docs for new developers in the docs/developer_guides/adr_process.md

How to test

1. Build docs to check no broken links as new developer Guide
2. Manual link/content check
3. Confirm CodeRabbit review of developer docs does not think they need a frontmatter status
4. Behavioral check - start a fresh Claude Code session in this repo and ask it to write an ADR. Confirm it follows the pointer in AGENTS.md → docs/agents/adr_process.md and produces output consistent with that doc (correct template, correct docs/adr/index.md/mkdocs.yml update steps) — rather than falling back on stale memory of the old inline instructions.
5. Skill dependency check: since .claude/skills/extract-adrs/SKILL.md doesn't reference AGENTS.md at all — its a docs status: active/stable/extracted gating logic that is self-contained, so trimming AGENTS.md can't break the automated extraction flow.