fix(password): don't print secret to a terminal on --password-get

[jamiesun]

Problem

--password-get=<key> printed the stored password in plaintext to stdout by default (\nPassword: <value>\n), leaving the secret in terminal scrollback / shoulder-surfing range. That print is largely unnecessary:

• sshx already uses the keyring internally (auto-fills sudo over stdin) — humans rarely need the raw value.
• --password-check already answers "does this key exist?" without revealing the value.
• The decorated Password: <value> output wasn't even clean for scripting.

Fix

Make --password-get TTY-aware:

• Interactive terminal → does not print the secret; confirms the key exists and shows how to pipe it.
• Piped / redirected stdout → emits only the raw value (no decoration, no trailing newline) for clean capture: PW=$(sshx --password-get=key) or sshx --password-get=key | pbcopy. The plaintext warning goes to stderr so it never pollutes the value.

No new flag; backward-compatible for scripts (output is actually cleaner now). The not-found error path is unchanged.

Verification

• gofmt/go vet clean; go test ./... passes; golangci-lint (v2) 0 issues.
• Manual probe with a throwaway keyring entry:
  • piped capture → exactly the raw value (10 bytes, no trailing newline);
  • warning lands on stderr only;
  • pty (script) run → secret not printed, only "exists" + pipe guidance;
  • missing key → password not found, exit 255.

Docs

Updated usage help, skills/sshx/SKILL.md, README.md, README_CN.md, the list-passwords hint (now points at --password-check), and a CHANGELOG ### Security note.