feat(web): split context injection into producer-declared forms

Injected context reached the transcript as one anonymous shape whose
expanded body was the whole message serialized as inline JSON, so the
escaping collapsed the only readable part — the model-facing prose —
into a single line.

`MessageSource` gains an optional producer-declared `form`, a small
semantic vocabulary of information shapes independent of `kind`: kind
says who produced the context, form says what shape it is, so several
producers may share one presentation. Two values ship.

`instructions` (workspace-context) lists the reconciled files above
their text and keeps the `<system-reminder>` framing verbatim, because
the framing is part of what the model read. `catalog` moves dsh-tool-skill
off the shared plugin kind onto a `skill-catalog` source carrying the
published name/description entries, and the body lists those instead of
re-parsing `<available_skills>` out of the prose. Catalog identity moves
with it: the republish digest now covers the durable entries, deleting
the text-slicing that recovered them from a logged message.

Everything else renders the opaque body — the model-facing text with its
real line breaks, then the remaining provenance as fields. That is the
documented default, not a leftover: a resumed, forked, or foreign log
must render whether or not its producer is mounted here, which is why
the classification lives in the durable source rather than a client-side
table keyed by producer.

packages/skill/tool-skill/README.md

@@ -10,7 +10,7 @@ Requires `ctx.agents`, `ctx.tools`, and `ctx.skills` (`inject: ['agents', 'tools
 
 At every `agent/step`, the plugin calls `ctx.skills.snapshot()` for the calling session's cwd, forwards the step abort signal to discovery, applies exact `skill` tool visibility, and renders the ordered `name` and `description` entries. When no prior catalog exists and that view is non-empty, it injects an initial durable user-role `<system-reminder>` before the request. Catalog messages contain only those summaries; skill bodies, paths, sources, providers, and `whenToUse` hints remain outside the catalog.
 
-The digest covers the exact rendered text between the `<available_skills>` tags. The plugin scans durable session events backwards without copying them and derives the comparison baseline from the newest recognizable visible catalog message it sourced. When the digest changes, `agent.inject()` records a durable user-role message containing the complete replacement catalog; an empty replacement explicitly retires earlier names. If no catalog remains visible but a recognizable historical catalog exists, compaction hid it and the next complete observation re-establishes the current catalog. An incomplete provider snapshot emits nothing and preserves the last-good model view for retry on the next step. If no prior catalog exists and the current view is empty, no tombstone is necessary.
+Every catalog message carries the `skill-catalog` source: a `catalog`-form context whose `entries` record exactly the `name` and `description` pairs it published, plus `update` on a replacement. The digest covers those durable entries, not the rendered prose, so the surrounding `<system-reminder>` framing — written for the model — cannot decide whether a republish is needed, and a consumer presenting the list never re-parses the `<available_skills>` block. The plugin scans durable session events backwards without copying them and derives the comparison baseline from the newest visible `skill-catalog` message. When the digest changes, `agent.inject()` records a durable user-role message containing the complete replacement catalog; an empty replacement explicitly retires earlier names. If no catalog remains visible but a recognizable historical catalog exists, compaction hid it and the next complete observation re-establishes the current catalog. An incomplete provider snapshot emits nothing and preserves the last-good model view for retry on the next step. If no prior catalog exists and the current view is empty, no tombstone is necessary.
 
 The catalog is omitted when no model-invocable skills are initially available, and also when that agent's tool view restricts away the shipped `skill` tool or resolves a same-name scoped shadow instead. Visibility changes participate in the digest, keeping prompt guidance, model-visible schema, and executable dispatch aligned.