perf(sync): overlap pre-unlock pull with the password prompt (#103, Tier 1)

[arimxyer]

Supersedes #108 (auto-closed when its base branch fix/102-content-hash-marker was deleted on #102's merge). #102 is now in main; this is rebased onto it and contains Tier-1 changes only.

What

Every sync-enabled read/write does a synchronous pre-unlock pull (CheckRemoteMetadata → rclone lsjson, a network round-trip) before the vault unlocks. Tier 1 hides that latency for password-prompt users by running the pull concurrently with the master-password prompt — where the human is already waiting — with no staleness tradeoff (unlike a TTL).

The ordering constraint

Unlock reads + decrypts vault.enc in one step, and a pull replaces that file, so the decrypt must run strictly after the pull. Only work that doesn't touch vault.enc may overlap it.

• New cmd/unlockVaultWithSync replaces the sequential syncPullBeforeUnlock(vs) + unlockVault(vs) pair at the 6 pull-path commands (add/delete/list/get/exec/update). usage/migrate, which never pulled, are unchanged.
• Password path: kick off SyncPull in a goroutine → prompt + read the password (stdin only) → join the channel on every return path (so rclone is never orphaned on a prompt error/Ctrl-C) → decrypt against the now-current file.
• Keychain path stays sequential: retrieving the keychain password never touches vault.enc, but there's no prompt to hide behind.

Scope: Tier 1 only — Tier 2 (the common case) is deferred to #103

Keychain is the default unlock and gets ~zero benefit here (no prompt to overlap). Tier 2 — overlapping the Argon2 KDF for keychain users — is the headline win and is intentionally split out (it's where the concurrency/crypto risk lives and needs its own -race treatment). Prerequisite verified: SaveVault/SaveVaultWithDEK only touch UpdatedAt, so salt, iterations, and wrappedDEK are invariant across normal saves → Tier 2 is viable with a post-join re-derive fallback. #103 stays open, scoped to Tier 2.

Tier 1 is still worth shipping alone: it's the goroutine+join skeleton Tier 2 builds on, and a real win for headless/CI/scripted runs where keychain is absent.

Correctness details

• Conflict re-surfacing: SyncPull's conflict warning can print mid-prompt (garbled) and read commands never re-echo it (only writes do, via SyncPush). New vault.SyncConflictDetected() lets the orchestrator re-print the conflict cleanly below the prompt after the join, so the "sync resolve" signal is never lost.
• RetrieveKeychainPassword: retrieves the master password from the keychain without decrypting (the seam enabling overlap). UnlockWithKeychain now calls it then Unlock — behavior unchanged.
• Feedback: no transient indicator during the overlap (it would corrupt the prompt line); the verbose pull line is a full line above the prompt. TUI stays sequential (low-frequency, interactive) — a deliberate scope call.

Tests

RetrieveKeychainPassword (not-enabled → ErrKeychainNotEnabled); SyncConflictDetected default; and a -race test driving the concurrent password branch end-to-end against a real sync-enabled, initialized vault. Full unit + integration suites green; vault+cmd pass go test -race; golangci-lint v2.5 clean.

Refs #103 (Tier 1; Tier 2 remains tracked in #103).

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