feat(cli): envelope-probe discoverability + kind-key refusal prose (#193 residue)

The --unwired help line names the whole-envelope starting move (start
from a bare {}), closing the 0106-F1 discoverability gap the narrowed
issue kept open; the wrong-kind refusal names the exact key a blind
author must fix ('the "kind" key must be "process"') instead of
describing the mismatch abstractly. The third original item (block-ids
vs field names in --vocabulary) is sufficiently served by the
envelope probe per the issue's narrowing comment.

RED-first (tdd-author handoff): both pins observed failing on the old
help text / prose.

Gates: workspace 1043/0, clippy -D warnings clean.

closes #193

crates/aura-cli/src/research_docs.rs

@@ -39,7 +39,7 @@ pub struct DocIntrospectCmd {
     /// Describe one block's typed slots
     #[arg(long, value_name = "ID")]
     pub block: Option<String>,
-    /// List the open slots of a (partial) document
+    /// List the open slots of a (partial) document — start from a bare {} to see the whole envelope
     #[arg(long, value_name = "FILE")]
     pub unwired: Option<PathBuf>,
     /// Print the content id of a valid document
@@ -112,7 +112,7 @@ pub(crate) fn doc_error_prose(what: &str, e: &DocError) -> String {
                 DocKind::Process => "process",
                 DocKind::Campaign => "campaign",
             };
-            format!("{what}: wrong document kind (expected \"{want}\")")
+            format!("{what}: the \"kind\" key must be \"{want}\"")
         }
         DocError::Malformed(msg) => format!("{what}: {msg}"),
     }