fix(web): address review findings on producer-declared context forms

- `catalogHistory` validated its durable read. `agent.session.events` is a
  JSONL/SQLite seed on resume or fork, and seed validation guarantees only a
  source object with a non-empty `kind`; a `skill-catalog` record with missing
  or wrongly shaped `entries` threw inside the step listener, failing every
  later turn of that session. It is now skipped as an unrecognizable record,
  the posture the replaced content digest had, with a regression test over six
  malformed shapes.
- The headless keyless smoke still filtered catalogs by the old plugin source,
  so the `built-bin-smoke` gate would not have found the catalog message.
- Entries record the published description unescaped. The pseudo-XML escaping
  belongs to the `<available_skills>` frame and is applied when rendering it,
  so a description containing `<` no longer reaches the card as `&lt;`.
  `escapeText` is injective, so republish semantics and the model-facing text
  are unchanged.
- Adjacent text blocks join with no separator, matching how provider adapters
  flatten them; the body no longer shows a line break the model never saw.
- Provenance fields are bounded like the text: an unknown producer may record
  an arbitrarily large value.
- Both readers are all-or-nothing, and the row's form marker reports what
  rendered rather than what was declared, so a partly unreadable record cannot
  present a confident but incomplete account.
- The catalog body consumes `update` as a replacement notice; the digest
  canonicalizes per entry as JSON, since every separator character is itself
  legal in a description.
- `core.md` documents the form axis with a `ContextForm` type-equiv block, and
  both projections assert the wiring they duplicate.

docs/core-data-structures/core.md

@@ -167,6 +167,32 @@ interface MessageSourceMap {
 }
 ```
 
+Provenance and shape are two independent axes. `kind` answers *who produced this*; the optional `form` a producer mixes in answers *what shape of information it is*, so several producers may share one presentation and one producer may emit more than one shape over a session. The vocabulary is semantic and grows one value at a time; an absent or unrecognized value is the documented default, presented as opaque content:
+
+```ts type-equiv
+/**
+ * What SHAPE of information a producer-supplied context carries, declared by
+ * the producer beside its provenance.
+ *
+ * `MessageSource.kind` answers *who produced this*; `form` answers *what kind
+ * of thing it is*, and the two axes are deliberately independent — several
+ * producers share one form (three snapshot producers today), and one producer
+ * may emit more than one form over a session.
+ *
+ * The vocabulary is SEMANTIC, never visual: a value states that the content is
+ * a file's instructions or a catalog of available items, and a consumer decides
+ * what that looks like. Colors, icons, ordering, and collapse defaults are the
+ * consumer's business and must not enter this union. It grows one value at a
+ * time as producers gain the structured fields their form needs; an absent or
+ * unknown value is the documented default, presented as opaque content.
+ */
+type ContextForm =
+  /** Instructions read out of workspace files the model is expected to follow. */
+  | 'instructions'
+  /** A catalog of items available in this session, republished as it changes. */
+  | 'catalog'
+```
+
 ## Streaming
 
 Adapters emit a raw **chunk** protocol; the loop logs the chunks (replay fidelity) while feeding the same chunks through a `BlockAssembler` to rebuild blocks and messages. `StreamChunk` is a closed discriminated union over `type` — `block-start`, `text-delta`, `reasoning-delta`, `tool-call-delta`, `block-end`, `usage`, `finish`.