feat(prompts): composeInstructions honors overrides — the P2 segment reconciliation

The other half-wired seam: Platform's settings 'agent.prompts' switches
had no consumer in the agent layer, and their effectivePromptSegments
mirrored the PRE-sources-v1 loader — the shared/ fragments dir (the
learner-files doctrine) was invisible to it, so a naive delegation would
have silently dropped the segment from both agents' prompts.

- src/core/prompt-segments.ts (NEW, pure): the mirror-layout path
  grammar + segment types, defined exactly once; core stays zero-IO.
- src/agent/prompt-segments.ts (NEW, the CANONICAL discovery): base +
  shared/ + per-agent fragments, override resolution, allSegmentPaths.
  Composition is an agent concern; the agent layer reads its own tree
  (the architecture law pins agent and store apart — the first direct
  refactor attempt tripped the enforced import/no-restricted-paths
  zone, hence the three-layer split).
- src/store/prompt-overrides.ts: slimmed to the override-file CRUD
  behind the core port; shares the core grammar. Its doc no longer
  claims to be the canonical mirror.
- teacher-agent: composeInstructions(kind, switches?) composes through
  effectivePromptSegments — a segment whose switch is ON and whose
  override file exists composes the override text. Switches-less calls
  use the cached pure-source composition (byte-identical to the old
  loader, pinned by test); AgentBuildOptions.promptSwitches threads
  effective.agent.prompts per turn, composing fresh so override-file
  writes flip the very next turn.
- server barrel: re-exports the agent module's resolution for the
  settings routes (server→agent, the lawful direction).

Tests: byte-identical source pin, override swap-in (on/off/file-absent),
shared/ overridability in both agents, agent-construction threading,
updated segment-list pins with shared/ in composition order. This file
keeps owning PROMPTS_OVERRIDE_DIR (parallel-test discipline).

Gates: 671/671, typecheck, lint, build.

src/core/index.ts

@@ -29,6 +29,7 @@ export * from "./layout.ts";
 export * from "./plan-tree.ts";
 export * from "./phases.ts";
 export * from "./pending.ts";
+export * from "./prompt-segments.ts";
 export * from "./math-delims.ts";
 export * from "./ui-message.ts";
 export * from "./tool-parts.ts";