refactor(docs): split dev-workflows into hub, verification, frontend

[EduPav]

Summary

docs/dev-workflows.md had grown to 461 lines, 288 of them (62%) in the ## Testing section alone. It was three documents with three different read triggers glued together, so an agent that only needed "what do I run before a PR" loaded all 461 lines.

Split three ways, and adds a guard so this kind of move can't silently break inbound anchors.

File	Lines	Read it when
docs/dev-workflows.md	461 → 124	Process hub: command index, per-task checklists, releases
docs/verification.md	289 (new)	A gate failed, or you need what a command actually checks
docs/frontend-conventions.md	82 (new)	Building or restyling a screen, wiring data fetching

Issue

None — no existing issue tracked this.

Scope

What deliberately stayed in the hub. The command index and "Review ownership" did not move: six docs link #commands, two GitHub templates cite Review ownership by name, and checkGateClaims asserts DOCUMENTED_COMMANDS are named in docs/dev-workflows.md specifically. Keeping both in place meant the guard needed no loosening.

Content moved verbatim and in original order, so the move is verifiable by diff instead of by rereading. Verified mechanically: every non-heading line of the old file is accounted for in the three new ones.

Two things fixed in passing, both consequences of the split:

• The pnpm check:agent-docs prose was mis-nested under ### Python dependency updates — unrelated content under the wrong heading. It is now its own ## Agent-docs check section, which also makes AGENTS.md §5's inbound link land on the actual content instead of a 288-line section.
• The chain note said "see 'Documentation Update' above", which is now a different file, so it became a real link.

New guard — scripts/agent-docs/doc-links.mjs. Only the SECURITY.md → AGENTS.md anchor was ever verified, so every other cross-doc anchor could rot in silence: GitHub serves the file and lands the reader at the top, and a dead anchor reads as a working link. The new check resolves every relative Markdown link and #anchor against real files and headings. It is dependency-free and offline — external http/mailto links are never fetched — and skips fenced code so a shell example isn't parsed as a link.

Running it on the pre-split tree surfaced one real pre-existing failure and a bug in anchorOf(): it collapsed runs of whitespace, but GitHub emits one hyphen per space, so punctuation dropped from a heading leaves a double hyphen (RELIABILITY.md#stateful-counters--durability-caveats). The doc was right; the slugger was wrong. Fixed, and the check registered in AGENTS.md §5.

Links repointed (all now machine-verified): AGENTS.md §5/§6/§8/§9/§10, README.md ×5, the five docs/features/*.md prerequisite links, the ci.yml header comment, and one tech-debt-tracker reference. The feature-doc links were also inaccurate before — they pointed at #commands for a prerequisite list that lives in ## Non-live verification.

Verification

Command	Result	Notes
pnpm verify	pass	exit 0; agent-docs check passed (110 checks), 187 backend + 164 frontend tests, 4 structural
pnpm verify:full	not run	needs live B2 credentials, a bound local server, and Playwright Chromium; this PR touches only Markdown and the dependency-free doc guard, so no browser or live-service behavior is in scope

Negative-tested the new guard rather than only confirming it green — broke an anchor and a file path in AGENTS.md and confirmed both fail with the real anchor list in the message, then restored:

- AGENTS.md:161 links to docs/verification.md#python-deps — expected a heading anchored
  #python-deps in docs/verification.md, actual anchors: ["verification","local-environments",...]
- AGENTS.md:176 links to docs/frontend-convention.md — expected a file at
  docs/frontend-convention.md, found nothing

AGENTS.md is 230/250 lines, still inside its enforced cap.

UI evidence

N/A - no UI changes.

Docs

• Updated relevant docs in this PR (see the doc update mapping in AGENTS.md section 9)

AGENTS.md §9 was the highest-risk edit: its one "Dev or testing process" row is how agents route doc writes, so it became three rows. A stale row there would send future changes to the wrong file, which is worse than a long file.

Risk / rollback

• Risk: low, and mostly one-way. Content is unchanged prose; the failure mode of a doc split is a broken inbound link, which the new guard now blocks in CI. Residual risk is external links (blog posts, bookmarks) pointing at moved dev-workflows.md anchors — nothing in-repo can vouch for those. Not considered a breaking change under the release policy: doc file paths aren't in the versioned surface.
• Rollback: revert the single commit. No migrations, config, or runtime code touched.

Review ownership

No CODEOWNERS in this repo, so no reviewer is auto-assigned — requesting manually.