[Feature] Add Forgejo source provider (v1) with repo-scoped search and required URL

[mbe24]

Problem

99problems currently supports GitHub, GitLab, Jira, and Bitbucket, but not Forgejo.

Users on Forgejo instances cannot use get for issue/PR context retrieval, which blocks adoption in self-hosted and Forgejo-centric environments.

Design

Implement Forgejo provider v1 with balanced parity:

1. search + ID fetch for issues and pull requests,
2. issue comments + PR review comments,
3. link hydration from timeline/dependencies/blocks.

Locked v1 boundaries:

1. Forgejo URL is required (--url or instance url),
2. search is repo-scoped only (repo:owner/repo / --repo required),
3. instance-wide cross-repo search is deferred.

Scope

• Add new source adapter under src/source/forgejo/.
• Wire platform selection and source construction for forgejo.
• Add FORGEJO_TOKEN env support in config/token resolution.
• Implement repo-scoped search mapping and ID fetch endpoints.
• Implement comments/review-comments/link hydration with parallelized branches.
• Update docs/examples (README/config/skill provider listings).

Boundary

• No cross-repo Forgejo search in this issue.
• No gitea alias in this phase.
• No behavior change for existing providers.

Acceptance Criteria

• 99problems get --platform forgejo --url <forgejo-base> --repo owner/repo -q "..." works.
• 99problems get --platform forgejo --url <forgejo-base> --repo owner/repo --id <n> --type issue|pr works.
• Issue comments and PR review comments are returned according to flags.
• Link metadata is hydrated best-effort from Forgejo timeline/dependency/blocking APIs.
• Missing Forgejo URL returns clear usage guidance.
• Existing provider tests remain green.

Context

Forgejo exposes compatible issue/PR APIs and review/timeline/dependency endpoints, making a focused v1 adapter feasible without broad architectural changes.

• https://codeberg.org/forgejo/forgejo/src/branch/forgejo/templates/swagger/v1_json.tmpl
• https://git.disroot.org/
• https://codeberg.org/forgejo/-/packages/container/forgejo/14.0.3
• https://codeberg.org/forgejo-contrib/delightful-forgejo#user-content-public-instances