docs: point the contributor checklists at docs that exist (BEA-191)

[ssowonny]

TL;DR

• Every PR here shipped with a checkbox nobody could tick.
• The template and CONTRIBUTING.md both demanded you update plugin/skills/beardrive/SKILL.md — a file deleted on purpose, in a plugin/ directory that doesn't exist.
• An untickable box trains people to tick without reading, which quietly devalues the three real boxes above it.
• Both lines now name the surfaces CLAUDE.md's own "Docs to keep in sync" section lists.
• Docs only. Two lines, no code, no new paths that don't exist.

The change

One substitution, applied in the two places the stale path survived:

  .github/PULL_REQUEST_TEMPLATE.md:10
- - [ ] CLI behavior changes updated both `README.md` and `plugin/skills/beardrive/SKILL.md`
+ - [ ] CLI behavior changes updated `README.md`, `INSTALL_FOR_AGENTS.md` and `web/docs/src/content/docs/`

  CONTRIBUTING.md:49-52
  - **Docs travel with behavior.** Changing CLI commands, flags, or output
-   means updating both `README.md` and `plugin/skills/beardrive/SKILL.md`
-   — the skill is what makes agents beardrive-aware and must match the
-   binary.
+   means updating `README.md`, `INSTALL_FOR_AGENTS.md`, and
+   `web/docs/src/content/docs/` — `INSTALL_FOR_AGENTS.md` is the runbook
+   agents follow to set themselves up, and `web/docs` is the end-user
+   reference published at docs.beardrive.ai.

The CONTRIBUTING justification clause goes with the path. "The skill is what makes agents beardrive-aware and must match the binary" described nothing that exists — the agent integration is internal/agenthooks plus INSTALL_FOR_AGENTS.md, which is what CLAUDE.md says too.

Why these three surfaces

They aren't a judgment call. CLAUDE.md's "Docs to keep in sync" section already names exactly them, so the edit is a substitution rather than an invention:

surface	what goes stale in it
README.md	flags, output formats, on-disk layout
INSTALL_FOR_AGENTS.md	the init / login / hooks flow agents follow
web/docs/src/content/docs/	reference/cli.md, reference/hub-config.md, reference/project-files.md, self-hosting/

web/docs/src/content/docs/ is a directory, not a file. Naming the directory keeps the checkbox tickable in ten seconds; naming its four stale-prone pages would have turned one box into five, which is the problem this PR is fixing.

What was checked

• git grep -n "plugin/skills" → no output. Those two lines were the only references repo-wide; nothing else pointed at the deleted tree.
• ls README.md INSTALL_FOR_AGENTS.md web/docs/src/content/docs/ → all three exist.
• PR template checkbox count and ordering unchanged: 4 before, 4 after.
• go build ./... && go vet ./... green. go test ./... green (see below) — no .go or frontend file is touched by this diff, so no npm run build, no static/ rebuild, no e2e, and no new test to add.
• Touches no package drawn in architecture/, so no diagram section.

What this doesn't do

The scope was deliberately two lines. It does not audit other stale paths elsewhere in the repo, restore a plugin/ directory, or rewrite the rest of either file. If more corpses are pointed at from contributor-facing docs, they'd need their own sweep — this PR only proves there are no others named plugin/skills.

Closes BEA-191.

Build session

cd $(git worktree list | grep infallible-kepler-9df580 | awk '{print $1}') && claude --resume 44ad41c1-c6fa-4bc2-b442-ef64a1814aa0

(Only works on the machine this branch was built on.)