feat(gitlab): add project wiki support to the GitLab connector

[MeEfe]

Add GitLab wiki connector

Closes #47.

GitLab stores project wikis separately from the repository, so this adds wiki
support to the existing GitLab connector. Add ?wiki=true to a gitlab: source
to sync a project's wiki instead of its code — each wiki page becomes a file in
the KB, preserving the page hierarchy from its slug.

Usage

oikb sync "gitlab:owner/repo?wiki=true" --kb-id <kb>
oikb sync "gitlab:group/subgroup/project?wiki=true" --kb-id <kb>   # subgroups
oikb sync "gitlab:42?wiki=true" --kb-id <kb>                        # numeric project ID

Uses the Project Wikis API (GET /projects/:id/wikis), reuses GITLAB_TOKEN /
GITLAB_URL, and adds no new dependency (httpx is already core).

What's included

• ?wiki=true flag - routes the GitLab connector to the wiki endpoint. Repo mode is byte-for-byte unchanged.
• Numeric project IDs - gitlab:42 / gitlab:42?wiki=true, used directly as the API :id. Handy when the namespace path is deep or awkward to encode.
• Subgroup paths - gitlab:group/subgroup/project?wiki=true, URL-encoding the full nested namespace.
• Older-GitLab fallback - uses with_content=1 on the list endpoint when available, and falls back to GET /wikis/:slug per page on instances that don't return content inline.

Design note

I extended the existing GitLab connector with a ?wiki=true source flag rather
than adding a separate connector, following the query-string option convention
already used by the Jira connector (jira:KEY?jql=...). This keeps the change
contained to gitlab.py and its source parser and leaves the repository code
path untouched. Happy to split it into a dedicated gitlab_wiki: connector
instead if you'd prefer.

Testing

Tested end-to-end against a self-hosted GitLab instance:

• Synced a 44-page wiki via the path form (gitlab:org/repo?wiki=true) and the numeric ID (gitlab:42?wiki=true).
• Through all three trigger paths: direct CLI (oikb sync <source>), the .oikb.yaml config file, and the daemon API (built a Docker image from this branch and ran it as the daemon).
• Verified the manifest (filename/path/checksum/size), nested wiki slugs (e.g. Administrative/Local-information.md → path Administrative), and that the request hits /projects/:id/wikis?with_content=1.

Verified with mocked (respx) requests during development: subgroup path encoding
(group%2Fsubgroup%2Fproject), the numeric-repo form, the older-GitLab per-page
fallback, and that repo mode is unchanged.