v2.2.0

This is a v2 minor release containing all fixes from PR #120 (the reviewed
commits in 6431cae..c77ffdf). It changes some default behavior and has a
vault-format compatibility requirement. The official exact-tag release
workflow creates the stable v2.2.0 Release with make_latest=false;
v2.1.0 remains GitHub latest until published-asset canaries pass and a separate
promotion is made. Ordinary ssm update stays within v2.

Upgrade before saving or publishing (read first)

• Vault format v2 encryption. New vaults use an authenticated monotonic
  generation. Older clients (v2.1.0 and earlier) cannot read a v2-format vault;
  upgrade all machines to v2.2.0 before any machine saves or publishes one.
  v2.2.0 continues to read old-format vaults.
• Conditional sync publication. The sync server now checks If-Match and
  If-None-Match preconditions on uploads. If two machines upload at once,
  only one succeeds and the other receives sync_conflict. A new client can
  still push to an old server, but that server cannot provide this protection.
  If the local remote.etag cache is missing while a remote vault exists, push
  first requires sync or pull.
• Deploy the new internal/syncserver before relying on the upload checks.

User-visible changes

• A push whose scope no longer matches an interrupted publication fails as
  sync_push_failed (exit 1), and it does not publish extra inventory.
• A downloaded vault that fails authentication is rejected and the existing
  connections.enc.prev is retained.
• A client that has never synchronized reports empty last_pull and
  last_sync values.
• password_cache and vim_keys settings are ignored; put to an existing
  remote directory fails.
• If the server returns an older vault (a rollback), the client reports
  sync_conflict; review it and use pull --adopt-remote to adopt it.
• When HOME cannot be determined, startup errors and instructs the operator
  to set HOME or SSM_CONFIG_DIR.
• Credential files passed with --password-file, --key-file, --secret, or
  --master-pass-file that are readable by other users produce a warning.
  Set SSM_NO_PERMISSION_WARNING=1 to suppress it; Windows does not warn.
• get directory downloads reject device files and remove special permission
  bits. Cloud clients reject cross-host or downgrade redirects.
• status reuses one unlock and completes faster.

There is no new error code and no request-schema or JSON-field breaking
change. The checked range has no request-schema file changes
(git diff 6431cae..c77ffdf --stat -- skills/agent-ssm/references/*.json is
empty); result and request additions remain additive.