Context hygiene at insert time: result ids, read stubs, the cap, recall

Harness spec 3.0.0 (context.md, loop.json `context`), in session/results.ts,
applied by the engine when a result goes into the conversation and never
afterwards:

- every result is headed by its id ([#0001]); a read also by the path as
  given, its lines, the file's length and the first 8 hex digits of the
  content's SHA-256; an empty file says so;
- a read of an unchanged file whose lines lie inside an earlier full read is
  a stub naming it; a list/glob/grep repeated with the same arguments and the
  same result is "Same result as #…";
- output over 400 lines or 40,000 characters keeps head and tail with the
  elided line; the full result is kept with the session (a new `results`
  table in sessions.db, gone with the session);
- edit, multiedit, write and apply_patch answer with their hunks under a
  bracket with the new content's hash; a new file is "Created, N lines.";
- recall (new built-in tool) shows any stored result, or a range of it, also
  after a resume; ids are never reused;
- compaction appends the table of the reads still in the conversation to the
  summary.

A stub only ever points at a result still in the conversation, so after a
compaction, an /undo or a rewind the file is read in full. compaction.prune
is gone: cutting old outputs in place threw the prompt cache away (an old
config's key is dropped with a warning). blocks/context.md is in the prompt
wherever recall is offered. Conformance runner for the `context` area (11
cases pass).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/tool/registry.ts

@@ -18,6 +18,7 @@ import { sessionSearchTool } from "./session_search.ts"
 import { skillManageTool, skillsListTool, skillViewTool } from "./skills.ts"
 import { settingsTool } from "./settings.ts"
 import { showTool } from "./show.ts"
+import { recallTool } from "./recall.ts"
 import { knowledgeGetTool, knowledgeSearchTool, noteManageTool, notesSearchTool, noteViewTool } from "./library.ts"
 import type { Tool } from "./tool.ts"
 import { writeTool } from "./write.ts"
@@ -58,6 +59,7 @@ export const BUILTIN_TOOLS: Tool[] = [
   settingsTool,
   viewImageTool,
   showTool,
+  recallTool,
   // Offered to subagents only (the spec's `offered: subagent`; app.ts SUBAGENT_ONLY).
   reportTool,
 ] as Tool[]