fix(mount): make --cache-dir a session-only override (#85)

[FROSADO]

Closes

Closes #85

Problem

A one-off mount --cache-dir /tmp/... permanently overwrote the account
config: step 5 of cmd/onecloudriver/mount.go always wrote
acc.Mount.CacheDir = config.CacheDir, even when the value came from the
CLI flag. Future mounts (e.g. systemd service restarts) then pointed at the
temporary path — and once that path was deleted, both user services
crash-looped (activating auto-restart, fusermount3 failures) until the
config was manually restored. The flag help read as a session override, but
the value silently became permanent.

Files involved

• cmd/onecloudriver/mount.go — --cache-dir is now a session-only
  override: when the flag is given, the value is used for the mount but
  never written back to acc.Mount.CacheDir. New pure helper
  buildPersistedMountConfig computes the config to save (unit-testable).
  When the flag is used, the startup line prints
  Cache: <dir> (session only, not saved to account config). Flag help and
  command Long text updated accordingly. All other flag overrides
  (--cache-ttl, --delta-interval, ...) keep being persisted as before.
• cmd/onecloudriver/mount_test.go (new) — buildPersistedMountConfig
  keeps the previously configured CacheDir when the flag is given (also
  when the persisted value is empty), persists it when no flag is given, and
  always persists the remaining scalar fields.
• docs/MANUAL.md / docs/MANUAL.es.md — cache flag table, persisted-config
  section and full parameter table document the exception.
• docs/ARCHITECTURE.md / docs/ARQUITECTURA.md / docs/ARQUITECTURA.es.md
  — --cache-dir row now notes "session only, never persisted".

Test plan

• go build ./...
• go vet ./cmd/onecloudriver/...
• go test ./cmd/onecloudriver/... -count=1 -short -race
• golangci-lint run ./cmd/onecloudriver/...
• Manual: mount with --cache-dir /tmp/x, verify the account JSON
  cacheDir is unchanged

Checklist

• Issue linked with Closes #N
• Branch follows the issue-N-slug convention
• Commit messages follow Conventional Commits
• Tests added/updated for the change
• CI checks pass (Build, Tests (unit + race), Tests (integration FUSE), Lint, Security Audit)

[coveralls]

coverage: 77.331% (+0.06%) from 77.276% — issue-85-session-only-cache-dir into main