refactor(agents): replace broken ZWSP sort prefixes with leading ASCII spaces

The ZWSP-based core agent sort prefixes silently failed to produce the
canonical sisyphus -> hephaestus -> prometheus -> atlas order.

Empirical testing of OpenCode's Agent.list() sort behavior shows that
Unicode collation treats zero-width characters as ignorable at the primary
level, so ZWSP-prefixed names sorted alphabetically with non-core agents
interleaved (e.g. Sisyphus, athena, Atlas, explore, Hephaestus, ...).

This commit replaces the ZWSP prefixes with leading ASCII spaces in
descending lengths (sisyphus=4, hephaestus=3, prometheus=2, atlas=1).
ASCII spaces sort reliably before alphabetic characters in localeCompare
under all locales and render correctly in every terminal.

Changes:
- AGENT_LIST_SORT_PREFIXES: ZWSP -> leading spaces (4-3-2-1 descending)
- stripAgentListSortPrefix: now strips both legacy ZWSP and new leading
  whitespace, preserving backward compatibility with existing sessions
- normalizeStoredAgentName / normalizeRegisteredAgentName: extract a
  shared stripSortPrefix helper that handles both prefix formats
- agent-config-handler: resolve user-provided default_agent display
  names through getAgentConfigKey before applying the runtime prefix,
  so configs like default_agent="Hephaestus - Deep Agent" are normalized
- agent-runtime-name-sort.test.ts: new regression test simulating
  OpenCode's exact sortBy logic (default_agent desc + name asc localeCompare)
  to verify canonical core agent order under randomised input permutations
- AGENTS.md: document the empirical finding that ZWSP was broken, why
  ASCII spaces work, and the descending prefix-length contract

Existing strip functions retain ZWSP support so legacy session state and
configs continue to resolve correctly without migration.

src/shared/agent-display-names.test.ts

@@ -194,11 +194,11 @@ describe("getAgentConfigKey", () => {
 })
 
 describe("getAgentListDisplayName", () => {
-  it("applies invisible stable-sort prefixes to the core agent list", () => {
-    expect(getAgentListDisplayName("sisyphus")).toBe("\u200BSisyphus - Ultraworker")
-    expect(getAgentListDisplayName("hephaestus")).toBe("\u200B\u200BHephaestus - Deep Agent")
-    expect(getAgentListDisplayName("prometheus")).toBe("\u200B\u200B\u200BPrometheus - Plan Builder")
-    expect(getAgentListDisplayName("atlas")).toBe("\u200B\u200B\u200B\u200BAtlas - Plan Executor")
+  it("applies leading-space stable-sort prefixes so OpenCode localeCompare yields canonical order", () => {
+    expect(getAgentListDisplayName("sisyphus")).toBe("    Sisyphus - Ultraworker")
+    expect(getAgentListDisplayName("hephaestus")).toBe("   Hephaestus - Deep Agent")
+    expect(getAgentListDisplayName("prometheus")).toBe("  Prometheus - Plan Builder")
+    expect(getAgentListDisplayName("atlas")).toBe(" Atlas - Plan Executor")
   })
 
   it("keeps non-core agents unprefixed for list display", () => {