GET a repository's tags answers `{ items, next_cursor }`, and every test that reads it is updated #5061

Intent: tags read like every other list. This is the other half of #5041.

Done:

• [ ] GET /api/v1/repos/:owner/:name/tags (src/http/api.ts, near line 2802) answers { items, next_cursor } with limit (default 50, max 200) and cursor checked the way the issue list checks them; a bad limit gets a 422
• [ ] newest first, as today; permission unchanged
• [ ] tests: the shape, paging to the end, and a bad limit refused
• [ ] every reader in src/ (mcp.ts, the terminal's refs screen, the web client) and test/ reads items. Run every file grep -rln '/tags' test lists, including test/api-tags-timing.test.ts, test/read-api.test.ts and test/parity.test.ts

The branches issue in this plan changes the same files, so rebase on whichever lands first. src/origins/providers/* stay as they are.