Add `pr merge` with all six Gitea methods, the three common-method
shorthands, `--auto`, `--delete-branch`, `--subject`, `--body`/`--body-file`,
and `--merge-commit-id` (required with and only valid for
`--method manually-merged`). Conflicting method flags and the
manually-merged/commit-id pairing are rejected as VALIDATION_ERROR before
any API call. An already-merged PR short-circuits to a `pull_request` entity
block with `merged_by`/`merged_at`; merge-blocked 405/409 responses surface
as VALIDATION_ERROR with update-branch/checkout remediation.
Add `pr update-branch` merging the base into the head via the update
endpoint with `--style <merge|rebase>` (default merge).
- CONVENTIONAL-COMMITS.md: vendored spec (v1.0.0); CLAUDE.md requires
agent-written commits to follow it
- tasks 0002-0020: remaining task breakdown of the gitea-axi spec
(0019 reframed around the three-tier test taxonomy: unit,
integration, end-to-end)
- spec, CONTEXT.md, and ADRs 0006/0007/0011: pending design
refinements from the planning sessions
