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.

examples/headless-agent/tests/keyless-smoke.e2e.ts

@@ -39,8 +39,7 @@ describe('headless-agent keyless smoke', () => {
     expect(stderr).toBe('')
     expect(events.some(event => event.type === 'tool/call' && event.data.name === 'bash')).toBe(true)
     const catalogMessage = events.find(event => event.type === 'user/message'
-      && event.data.source.kind === 'plugin'
-      && event.data.source.plugin === 'dsh-tool-skill')
+      && event.data.source.kind === 'skill-catalog')
     const catalog = catalogMessage?.type === 'user/message'
       ? catalogMessage.data.content.filter(block => block.type === 'text').map(block => block.text).join('\n')
       : ''