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/prompt/assemble.ts

@@ -254,6 +254,8 @@ export interface PromptContext {
   /** The library block (blocks/library.md): set when knowledge tools are offered or the instance
    *  library is in use; `scope` the sentence naming the bases in scope, or "" for all of them. */
   library?: { scope: string }
+  /** recall is offered: the tool results carry ids and may be stubs (blocks/context.md, spec 3.0.0). */
+  context?: boolean
   /** Prototypes are offered (blocks/prototypes.md); `sources` the sentence saying what a page
    *  may load, or "" where nothing says. */
   prototypes?: { sources: string }
@@ -337,6 +339,8 @@ export function assembleSystem(ctx: PromptContext): string {
   else if (ctx.manageSkills && !ctx.subagent) sections.push(promptText("blocks/skills-empty.md"))
   // The library and prototypes (shared blocks): the library whenever its tools are offered, named
   // to the bases in scope; prototypes only where they are offered, with where the page is seen.
+  // How tool results are kept short (ids, stubs, recall): wherever recall is offered.
+  if (ctx.context) sections.push(promptText("blocks/context.md"))
   if (ctx.library && !ctx.subagent) sections.push(fillPrompt(promptText("blocks/library.md"), { knowledge_scope: ctx.library.scope }))
   if (ctx.prototypes && !ctx.subagent) sections.push(fillPrompt(promptText("blocks/prototypes.md"), { prototype_sources: ctx.prototypes.sources, prototype_surface: PROTOTYPE_SURFACE }))