Implement the Forgejo planning pull request adapter

[rochecompaan]

Part of #180.

Blocked by: #184

Goal

Implement Forgejo support behind the provider-neutral planning pull request interface.

Scope

• Resolve target and configured push-remote repository identities through Forgejo.
• Support cross-owner heads and remotes other than origin.
• Implement create, exact discovery, and get through tea api.
• Fetch all list pages without returning partial results.
• Distinguish exact API 404 responses from authentication, rate-limit, transport, and malformed-response failures.
• Add normalization, pagination, and command contract tests.

Out of scope

• GitHub support.
• Planning workspace or pipeline integration.

Acceptance criteria

• The adapter satisfies the foundation interface.
• Nested target and head repository identities are strictly validated.
• Pagination continues until a short page is returned.
• Only an exact API 404 becomes PullRequestNotFoundError.
• The adapter is not yet wired into run-once.

[rochecompaan]

This was generated by AI during triage.

Triage Notes

This issue is clear and suitable for automation, but its provider-neutral foundation is still open. Re-evaluate it once that prerequisite is complete.

Blocked by: #184