GeezerVison

Chats that belong to a chat project now get the project's skills, next
to personal, organization and workspace skills. They show up in the
model's skill list, load through `read_skill`, and appear in a "Project
skills" group in the `/` menu.

- `coderd/x/skills` adds the `project` source. `MergeSkills` now takes a
list of sources in merge order (personal, organization, project,
workspace). Bare names stay bare unless two sources share a name, in
which case they get `project/<name>` style aliases.
- chatd loads project skills only when the chat owner owns the project
or the project is shared with them. Role grants don't count. This is the
same `usableChatProject` check that project instructions and memory use,
and one check per turn covers both instructions and skills. Disabled
skills are skipped. A failed load logs a warning and the turn continues
without them. Subagents use their root chat's project; that lookup is
now a helper shared with project instructions.
- The site loads project skills for the composer on the project page and
in project chats, behind the `chat-projects` experiment. `/clear` and
`/compact` defer to a project skill with the same name, as they already
do for other sources. When the user can't use the project, the site
treats it as no project skills, like chatd, so those commands keep
working and the `/` menu shows no retry prompt for project skills. That
covers a 403 or 404 from the project skills request and a project
missing from the user's project list, which holds only owned and shared
projects, so an administrator role that can read the project doesn't
count. Other failures are retried and leave the list unknown. A cached
project list is also unknown while it refetches or after a failed
refetch, so a share revoked since it was cached keeps those commands
pending instead of deferring to stale project skills.
- Docs gain a "Project skills" section in the extending agents page.
`coderd/x/chatd/ARCHITECTURE.md` gains a paragraph on project skills.

Dogfood UAT on this branch passed: project skills appear in the project
page `/` menu and in project chats, `read_skill` loads them, disabled
and deleted skills report not found, chats outside the project don't see
them, and a subagent gets them through its root chat. The site reads
project skills from the chat's own `project_id`, so subagent chats show
them in the composer once sub-chats carry their root's project (#30683).

| Project page `/` menu | `read_skill` in a project chat |
| --- | --- |
| <img alt="Project skills group in the project page slash menu"
src="https://raw.githubusercontent.com/coder/coder/assets/pr-30838-project-skills/pr/project-page-slash-menu.png"
width="600" /> | <img alt="read_skill loading a project skill in a
project chat"
src="https://raw.githubusercontent.com/coder/coder/assets/pr-30838-project-skills/pr/project-chat-read-skill.png"
width="600" /> |

Stack (merge top to bottom):
- #30838 👈
- #30854
- #30855
- #30856
- #30861
- #30880
- #30881
- #30882
- #30884
- #31122

…ed skill helpers (#30854)

Pulls the skill form body out of `SkillEditor` into a `SkillEditorForm`
with no dialog wrapper, and moves the row helpers (download, delete
confirmation, form values from a skill) out of
`SkillsTable`/`SkillsTableView` so the project skills dialog in the next
PR can reuse them. `SkillEnabledSwitch` gains a `readOnly` prop.

The personal and organization skill pages behave the same, with two
small fixes that came along: Save is locked while a submit is in flight,
so a double click no longer sends two create requests, and the Body
placeholder renders its line breaks.

Stack (merge top to bottom):
- #30826
- #30827
- #30838
- #30840
- #30854 👈
- #30855
- #30856
- #30861
- #30880
- #30881
- #30882
- #30884
- #31122

Adds `ProjectSkillsDialog`, the dialog for managing a chat project's
skills, plus the API client methods and react-query factories for the
project skill routes from #30827. Nothing opens the dialog yet; the next
PR adds the entry point in the project page Details panel.

- List view: "Project skills" with each skill's name, description, an
enabled switch and last updated time, a row menu with Edit (View when
read-only), Download and Delete, and a "No skills added yet" empty
state.
- New and Edit replace the list inside the same dialog and reuse
`SkillEditorForm`, including paste-to-import and file upload. Name is
locked when editing, and Escape asks before discarding unsaved edits.
- Without update permission the dialog is read-only: no New, Delete or
Save, and fields are disabled.
- Create, update and delete invalidate the project skill list, so the
composer `/` menu picks up changes too. A 409 (duplicate name or the
100-skill cap) is shown in the form and refreshes the list.

Stack (merge top to bottom):
- #30826
- #30827
- #30838
- #30840
- #30854
- #30855 👈
- #30856
- #30861
- #30880
- #30881
- #30882
- #30884
- #31122