perf(sync): cut sync-related startup latency — drop dead --hash, feedback, --offline

[arimxyer]

Closes #96.

Cuts the sync-related startup latency (and makes it visible), without touching the
change-detection model. Three low-risk pieces, plus review-driven fixes.

What changed

1. Drop the dead rclone lsjson --hash (internal/sync/sync.go)
CheckRemoteMetadata requested backend hashes that were never deserialized
(RemoteFileInfo has no hash field) and never used by any decision — the skip
check uses ModTime+Size and conflict detection uses a local sha256. On many
backends --hash forces extra server-side work, so removing it makes the
pre-unlock probe cheaper with zero behavior change. (Note: the backend hash is
the wrong tool to keep — see #102 for real content-hash change detection.)

2. Feedback by default (cmd/helpers.go)
The pre-unlock probe is a synchronous network round-trip; previously it was silent
unless --verbose. Now a transient Checking remote... indicator shows on
stderr (cleared with CR + ANSI erase, same as the existing Syncing...),
only when sync is enabled and not offline. stdout stays byte-clean for pipes.

3. --offline global flag (cmd/root.go)
Skips cloud sync for one command — run fully local. (--no-sync is taken by
init, so it's --offline.) It skips both the pre-unlock pull and the
post-command push: skipping only the pull would be unsafe, since SmartPush has
no independent remote-conflict check and could blind-overwrite a newer remote.
Offline changes simply sync on the next online run.

Review-driven fixes (second commit)

An adversarial review (3 lenses: sync-correctness, output-hygiene, scope/tests)
surfaced one medium issue and some polish:

• TUI ignored --offline (medium): runTUI called SyncPull/SyncPush
  directly, so pass-cli --offline (the default TUI launch) and
  pass-cli tui --offline still hit the network. Both now route through the
  gated helpers, so the TUI honors --offline and matches CLI feedback. (The
  unreferenced cmd/tui/main.go:Run is dead code — left untouched.)
• Docs: --offline added to the command-reference Global Options table and
  the sync guide's Offline Operation section.
• Test: added TestSyncPushAfterCommand_FeedbackOnStderr locking the normal
  (sync-enabled, non-offline) push path.

Verification

• gofmt -l, go vet, go build, golangci-lint (v2.5.0): clean
• go test ./...: green
• go test -race ./internal/sync/... ./cmd/...: green, no data races
• go test -tags=integration ./test/integration/...: green (55s)
• Manual smoke (built binary, sync-enabled config w/ a bogus remote): list
  hides usernames by default, list -q prints bare names, get --quiet returns
  only the value; online runs show the Checking remote... probe feedback;
  --offline skips the probe entirely and still returns the value.
• TUI fix verified by wiring (the two live sync sites in runTUI now go
  through the offline-gated, unit-tested helpers) rather than a headless TUI run,
  since the TUI is interactive.

Known minor (deferred, not blocking)

• The transient Checking remote... indicator clears cleanly on the happy path,
  but on the sync-failure path (rclone error / network down for a sync-enabled
  user) the internal warning glues onto it on a single stderr line. Cosmetic,
  stderr-only, and the user is being told sync failed anyway. Clean fix (a
  follow-up) is in the sync layer: prefix SmartPull's warnings with the clear.
• No isatty guard on the transient indicators (matches the pre-existing
  Syncing... pattern); offline: true in config disables sync persistently
  (exact parity with --verbose's viper binding).

Follow-ups (rebase on top of this)

• perf(sync): overlap pre-unlock remote probe with unlock to hide latency (split from #96) #103 — concurrent probe/unlock (the split-out piece that overlaps the probe
  with unlock; needs salt-invariance verification + -race).
• fix(sync): modtime+size change-detection can miss same-size+same-modtime remote edits (risk of silent clobber) #102 — content-hash change detection (replaces the ModTime+Size heuristic).

🤖 Generated with Claude Code