feat(cli): --json flag, pr diff/checkout, commit view, milestone create #16

Summary

Closes 5 of the 8 tickets from the CLI gap audit. The other 3 (#14, #15, #16) are blocked on new server endpoints and stay open.

What’s in

#	Feature	Notes
#10	`–json` flag (global)	Applied to pr/issue/epic/ci/milestone/repo/branch/commit list+view. Process-global atomic so it threads without changing every fn sig. Other commands (label, ssh-key, requirement, release, deploy, agent, board, runner) remain table-only — follow-up.
#11	`anvil pr diff `	Resolves base/head via GET /pulls/:n, then `git fetch` + `git diff origin/base…origin/head`. `–name-only` and `–remote` flags.
#12	`anvil pr checkout `	Mirrors `gh pr checkout`: refuses if working tree dirty, fetches, `git checkout -B origin/`.
#13	`anvil milestone create`	Server already has POST /milestones. Other CRUD verbs (edit/close/reopen/delete) need server endpoints — blocked. `issue milestones` and `issue create-milestone` now emit deprecation warnings + delegate.
#17	`anvil commit view `	Wraps `git show –stat –no-patch` (`–diff` to include patch).

What’s blocked (still open)

#14 `anvil pr rebase ` — needs `POST /pulls/:n/rebase` on server
#15 `anvil branch` parity (create / delete / protect / unprotect) — needs branch CRUD + protection endpoints on server
#16 `anvil status` / inbox — likely needs an aggregation endpoint

Test plan

`cargo test` — 39/39 pass (11 new: 3 output + 4 pr + 2 commit + 2 milestone)
`cargo clippy –all-targets – -D warnings` clean
`cargo fmt` clean
Live verification against prod:
- `anvil pr list fangorn/anvil-cli –json` returns valid JSON (parseable with jq)
- `anvil pr view 13 –repo fangorn/anvil-cli –json` returns valid JSON
- `anvil pr –help` shows `diff` and `checkout` verbs
- `anvil commit –help` shows `view`
- `anvil milestone –help` shows `create`

Notes

The `–json` flag is global and additive: existing scripts unaffected. The deprecated `issue milestones` / `issue create-milestone` aliases stay functional but emit a one-line warning to stderr.

Closes #10, #11, #12, #13, #17