Make applied skills visible in collapsed work activity

[OrlovAlexei]

Problem

T3 collapses a turn's tool activity into one line, e.g. Ran 3 commands and used 2 tools. A skill counts as one of those tools, so its name is only visible after expanding the group. For a lot of turns, "did it actually apply the skill?" is the one thing worth confirming, and confirming it costs an expand every time.

The signal is already in the stream: Claude and OpenCode send an explicit skill tool call, Codex loads a skill by reading its SKILL.md (and its own TUI labels that read with the skill name), Antigravity flags the read. T3 flattens all of it into a generic tool count.

Proposal

Treat an applied skill as its own kind of activity, not one more tool call:
normalize each provider's signal into one field on the activity payload, give it its own row label and icon, and name applied skills in the collapsed summary outside the tool count — Ran 1 command, used 1 tool, and Skills: first-skill-name, second-skill-name.

Never inferred from the assistant's prose; per-provider decision, including "no usable signal here" where that is the honest answer.

The open question

For providers without an explicit skill tool, the only signal is the read of SKILL.md. Codex's docs call loading that file the activation echanism — but reading it to review or edit it looks identical, so counting it will sometimes report a skill that was never applied. Three options, and I'd rather have a maintainer's call than guess:

1. Count the read, matching what Codex's TUI already shows.
2. Explicit tool calls only — Codex, Cursor and Grok then show nothing.
3. Match the read against the skill catalog the adapter already discovers.

Scope

Small: one extra field on an existing payload plus the existing summary line. No new event type, settings toggle, or UI surface. Not covered: $skill-name in Codex is sent as text and expanded natively, so no activity appears to label.

Related: #6756 asks to preserve the trace's expanded state. Same pain from the other side — this is about not having to expand at all to see that a skill ran.