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";

src/core/prompt-segments.ts

@@ -0,0 +1,47 @@
+/**
+ * The pure prompt-segment grammar (P2 reconciliation, 2026-09-19):
+ * segment-path validation + the shared segment types, ZERO fs. Discovery
+ * (reading the agent's prompt tree) lives in the agent layer
+ * (src/agent/prompt-segments.ts — the canonical loader composeInstructions
+ * composes through); the override-file CRUD lives in the store adapter
+ * (src/store/prompt-overrides.ts). Both import THIS grammar so the
+ * mirror-layout rules are defined exactly once.
+ *
+ * Path grammar: "system-prompt.md" (the shared base), "shared/<fragment>"
+ * (sources v1: fragments composed by BOTH agents), "prober/<fragment>" /
+ * "teacher/<fragment>" — where <fragment> is numeric-prefixed + .md
+ * (e.g. 20-teaching.md). POSIX-relative, no traversal, no nesting.
+ */
+
+/** A prompt fragment: numeric order key + .md (e.g. 20-teaching.md). Nothing else loads. */
+export const FRAGMENT_RE = /^\d+-[\w.-]+\.md$/;
+
+/** Which agent's prompt directory to compose. */
+export type PromptAgent = "prober" | "teacher";
+
+/** One segment of one agent's composed prompt. */
+export interface PromptSegmentInfo {
+	/** The override/store key — mirrors the source layout ("system-prompt.md",
+	 *  "shared/05-learner-files.md", "teacher/20-teaching.md"). Stable across
+	 *  agents: the base and shared/ fragments use the SAME path in both
+	 *  agents' lists. */
+	path: string;
+	/** The source text (what the segment runs without an override). */
+	sourceText: string;
+	/** The user's override text, or null when no file exists on disk. */
+	overrideText: string | null;
+	/** Effective state: the settings switch AND the file both present. */
+	enabled: boolean;
+	/** True for segments BOTH agents compose — the base and shared/ fragments. */
+	shared: boolean;
+}
+
+/** Validate a segment path against the mirror-layout rules (see module
+ *  doc). Exported for the routes, which also check membership in the
+ *  CURRENT source segment list. */
+export function isValidSegmentPath(segmentPath: string): boolean {
+	if (segmentPath === "system-prompt.md") return true;
+	const m = /^(prober|teacher|shared)\/([\w.-]+\.md)$/.exec(segmentPath);
+	if (!m || m[2] === undefined) return false;
+	return FRAGMENT_RE.test(m[2]);
+}