feat(skills/linear): add Documents support + Python helper script (#20752)

* feat(skills/linear): add Documents support + Python helper script

The bundled Linear skill (PR #1230) covered issues, projects, teams, and
workflow states via curl. It had no coverage for Linear's Documents API,
so fetching an RFC/doc from a linear.app URL required hand-writing
GraphQL against an underdocumented schema.

Adds:
- Documents section in SKILL.md explaining slugId extraction from URLs,
  the contentState (markdown) vs contentState (ProseMirror) split, and
  four canonical curl examples (fetch by slugId, fetch by UUID, list
  recent, title-search).
- scripts/linear_api.py — stdlib-only Python CLI wrapping the most
  common operations (whoami, list-teams, list/get/search/create/update
  issues, add-comment, update-status, list/get/search documents, raw
  GraphQL passthrough). Zero deps, reads LINEAR_API_KEY from env.

Auth header quirk (personal key takes bare $LINEAR_API_KEY, no Bearer
prefix) is already documented in the skill.

Found during RFC review: the existing skill's lack of document support
forced falling back to the browser (which hit Linear's login wall).
Also fixes a schema gotcha — the Document field is `contentState`, not
`contentData` (which returns 400).

Tested end-to-end against the production API:
  python3 linear_api.py whoami
  python3 linear_api.py get-document 38359beef67c
Both return expected payloads.

* fix(skills/linear): point LINEAR_API_KEY setup to the correct page

The org-level Settings > API page (/settings/api) only shows OAuth apps
and workspace-member keys. Personal API keys live under Account,
Security, access (/settings/account/security). Update both the setup
link in config.py (shown during hermes setup) and the setup step in
SKILL.md so users land on the page that can create a personal key.

hermes_cli/config.py

@@ -1944,7 +1944,7 @@ OPTIONAL_ENV_VARS = {
     "LINEAR_API_KEY": {
         "description": "Linear personal API key (used by the `linear` skill)",
         "prompt": "Linear API key",
-        "url": "https://linear.app/settings/api",
+        "url": "https://linear.app/settings/account/security",
         "password": True,
         "category": "skill",
         "advanced": True,