Merge origin/master into skill system branch

Use the new filesystem seam for skill file reads and system skill writes when ctx.fs is available, and include the skill tool in the generated tool catalog.

packages/core/agent-loop/README.md

@@ -53,6 +53,8 @@ forever:
     STEP loop:
       drain steering
       assembly = systemPrompt.assemble()
+      await serial agent/pre-step        ⟵ surface mutation (compaction) outside the step
+      session('step/start')
       request = waterfall agent/request
       stream llm.stream(request) → session('assistant/chunk')
       message = waterfall agent/step-result
@@ -74,8 +76,8 @@ Cancellation: `agent.cancel()` is the single public stop primitive — it clears
 ### What is NOT here
 
 Everything that goes beyond "call the model, run the tools, repeat" belongs to plugins listening on the event taxonomy:
-- Hooks: `agent/request`, `agent/step-result`, `tools/execute`, `agent/turn-continuation`
-- Compaction: `agent/request`
+- Hooks: `agent/pre-step`, `agent/request`, `agent/step-result`, `tools/execute`, `agent/turn-continuation`
+- Compaction: `agent/pre-step`
 - Sandbox, permission, plan mode: `tools/execute`
 - Sub-agents: implemented outside the loop as `ctx.subagents` providers; in-process providers use `ctx.agents.create()` and owned `AgentHandle` teardown, while child streaming/progress and background/poll collection remain deferred.
 - Persistence: `session/event` + `session/flush`