feat(cli): default to interactive passphrase when no key source given

Previously, invoking fcry without any of --raw-key, --passphrase, or
--passphrase-env produced a hard error ("must provide one of ..."). The
common, secure case (interactive TTY passphrase) thus required an
explicit flag, while the dangerous case (--raw-key on the command line)
was equally accessible.

Make the secure path the default: if no key source is specified, fall
back to PassphraseSource::Tty, which prompts on the terminal and runs
argon2id on encrypt. Explicit --passphrase still works and is now
redundant for the default invocation; --raw-key and --passphrase-env
remain unchanged and still suppress the default.

The previous "must provide one of ..." error path becomes unreachable
and is removed: the only way pw_src is None is when raw_key_str is Some,
which is handled by the existing encrypt/decrypt branches.

User-visible change: `fcry -i foo -o foo.enc` now prompts for a
passphrase instead of erroring out. Scripts that relied on the error to
detect missing arguments will instead block on a TTY read; non-TTY
callers should continue to pass --passphrase-env or --raw-key
explicitly.

Test Plan:
- `fcry -i plain -o plain.enc` prompts twice (passphrase + confirm),
  then `fcry -d -i plain.enc -o plain.out` prompts once and round-trips.
- `fcry --raw-key $(head -c32 /dev/urandom | base64) ...` still works
  and does not prompt.
- `PW=hunter2 fcry --passphrase-env PW ...` still works and does not
  prompt.
- `fcry --passphrase --raw-key ...` still rejected by clap
  (conflicts_with_all).

src/main.rs

@@ -39,6 +39,7 @@ struct Cli {
     raw_key: Option<Zeroizing<String>>,
 
     /// Read passphrase interactively (terminal). Implies argon2id KDF on encrypt.
+    /// This is the default when no key source is specified.
     #[clap(short, long)]
     passphrase: bool,
 
@@ -131,8 +132,13 @@ fn run(mut cli: Cli) -> Result<(), FcryError> {
     let raw_key_str: Option<Zeroizing<String>> = cli.raw_key.take();
     let pw_src: Option<PassphraseSource> = if cli.passphrase {
         Some(PassphraseSource::Tty)
+    } else if let Some(var) = cli.passphrase_env.take() {
+        Some(PassphraseSource::EnvVar(var))
+    } else if raw_key_str.is_none() {
+        // Default to interactive TTY passphrase when no key source is given.
+        Some(PassphraseSource::Tty)
     } else {
-        cli.passphrase_env.take().map(PassphraseSource::EnvVar)
+        None
     };
 
     let decrypt_mode = cli.decrypt;
@@ -144,12 +150,6 @@ fn run(mut cli: Cli) -> Result<(), FcryError> {
     let argon_parallelism = cli.argon_parallelism;
     drop(cli);
 
-    if pw_src.is_none() && raw_key_str.is_none() {
-        return Err(FcryError::Format(
-            "must provide one of --raw-key, --passphrase, --passphrase-env".into(),
-        ));
-    }
-
     if decrypt_mode {
         let raw_key = match raw_key_str.as_deref() {
             Some(s) => Some(parse_raw_key(s)?),