feat(cli): global --repo flag (every subcommand inherits)

[gfargo]

Follow-up to #948 which added `--repo` to `coco ui` + `coco log` only. Contributor asked to lift it so `commit` / `review` / `recap` / `changelog` all inherit too — particularly for scripts and editor / shell wrappers that want to drive coco non-interactively against arbitrary repos.

What this PR adds

• `--repo ` (alias `--cwd`) declared globally at the yargs root (`y.option('repo', { ..., global: true })`). Every subcommand inherits automatically; `coco --help` and per-subcommand help both surface it.
• `repo?: string` lifted to `BaseArgvOptions` so the TypeScript layer matches — every command's argv type sees the field through normal inheritance.
• Shared `applyRepoFlag(argv)` helper at `src/commands/utils/applyRepoFlag.ts`. Encapsulates "resolve path → chdir → getRepo(repoPath)" so every handler does it identically. Returns the bound SimpleGit instance for direct use.

Handlers updated

• `commit`
• `recap`
• `changelog`
• `review`
• `ui` (replaces feat(cli): --repo <dir> on coco ui + coco log to target an arbitrary repo #948's inline implementation)
• `log` (replaces feat(cli): --repo <dir> on coco ui + coco log to target an arbitrary repo #948's inline implementation)

The per-command `--repo` declarations from #948 are removed in favor of the single global declaration. Behavior unchanged; just deduplicated.

Why chdir + bound git + global flag together

A repo target isn't a single value — it touches three independent subsystems:

1. simple-git's baseDir — controls where git operations run
2. `process.cwd()` — what every `findUp` config-discovery walks from
3. Path resolution — relative paths in argv (e.g. `--branch`) interpret against cwd

Setting only one would create "coco operates on repo A but reads config from repo B" surprises. `applyRepoFlag` does all three in one shot so the rest of each handler can ignore the question.

Now usable across the board

```bash

Generate a commit message for a different repo

coco commit --repo ~/work/api

Review a PR-bound branch in another project

coco review --repo ~/work/api --branch feat/new

Recap a different repo's last week

coco recap --repo ~/work/api --since 1w

Generate changelog for another project

coco changelog --repo ~/work/api --branch main

Open workstation against any directory (#948)

coco ui --repo ~/work/api

Inspect another repo's log (#948)

coco log --repo ~/work/api --limit 10
```

Compatibility

Users on the `cd && coco ...` path see zero change — `argv.repo === undefined` makes `applyRepoFlag` skip the chdir and call `getRepo()` with no baseDir, preserving original behavior exactly. `--cwd` alias preserved from #948.

Test plan

• `npm run lint` clean
• `tsc --noEmit` clean
• `npm run test:jest` — 1681 tests pass (+4 from new applyRepoFlag tests)
• Verified end-to-end: `coco log --repo /tmp/scenario --limit 5` from coco repo cwd shows scenario commits
• Verified `coco --help` lists the global flag

Out of scope

• `commit --repo` doesn't auto-stage anything in the target repo — the user still controls staging via the usual `git add` workflow before invoking. Same for the other commands. `--repo` just retargets where commands READ from / WRITE to.
• Cross-repo workflows (e.g. "commit in repo A using a draft from repo B") are still out of scope — single `--repo` flag means one target per invocation. Could lift later if a real use case shows up.