feat(cli): add 'gander watch <file>' subcommand as alias for 'share --watch'

[scott]

Summary

• Adds gander watch <file.md> as a thin top-level shorthand for gander share <file.md> --watch (the canonical "live-share to gander.md on every save" flow).
• New runWatchCmd in share.go delegates to the existing runShareWithCtx(..., ["--watch", file]), so auth, mapping writes, fsnotify + debounce + PUT-to-gandermd, and "Already shared … refreshing in place." are reused unchanged.
• A header note in watch.go clarifies that the local runWatch (preview path, gander <file> --watch) and the new remote runWatchCmd (share path, gander watch <file>) are deliberately separate code paths despite sharing the watch name.
• Help/usage: watch is listed under the authed block in printUsage and the unauthed hint now mentions it alongside share / remove / list / manage / auth.
• Bash + zsh completions include the new watch subcommand.
• README leads with gander watch README.md as the headline live-share example; gander share --watch is kept as the explicit-flag equivalent.
• Man page (man/man1/gander.1) gains a synopsis entry, a Watch section, and an example. The existing TestManPageExistsAndRenders was extended to assert gander watch is rendered.

Related Issue

Closes #65

Testing

• go test ./... — all green, including three new tests:
  • TestWatchCmdEqualsShareWatch — end-to-end: runWatchCmdWithCtx issues the same POST /api/shares body (watch: true), opens the share, then PUTs updates on save against a httptest.Server. Parallel to TestShareWatchFlowEndToEnd.
  • TestWatchCmdRequiresAuth — with no token in ~/.gander, returns requireAuth's "not signed up" error and makes zero network calls.
  • TestWatchCmdRejectsBadArgs — rejects zero and two positional args.
• go vet ./... — clean.
• go build ./... — clean.
• CGO_ENABLED=0 go build -trimpath -ldflags "-X main.Version=…" -o gander . — produces a working binary; gander --help shows the new authed line and gander watch (with no args) prints the usage error.
• Man page still lints clean with mandoc -Tlint.

Notes

• runWatchCmd is paired with a runWatchCmdWithCtx variant (mirroring the existing runShare / runShareWithCtx split) so tests can cancel via context.
• No server changes — gandermd accepts the same POST /api/shares body the CLI has been sending; the alias is purely a CLI-side ergonomic.
• Coordinated landing for the docs/landing-page mirror will be tracked separately per the issue's "Pairing" note.