fix(agents): backfill ZWSP runtime names for agents missing name field

agent-key-remapper now unconditionally injects getAgentRuntimeName()
into the name field, ensuring OpenCode's localeCompare sort preserves
canonical core agent order even when builtin configs omit name.

Also fixes 3 pre-existing test failures where builtinSisyphusConfig
expected the original name to survive remapping.

src/plugin-handlers/AGENTS.md

@@ -6,26 +6,30 @@
 
 The canonical agent order is **sisyphus → hephaestus → prometheus → atlas**.
 
-This order is enforced by `CANONICAL_CORE_AGENT_ORDER` in `agent-priority-order.ts` and MUST NOT be changed.
+This order is enforced via two mechanisms working together:
+1. `CANONICAL_CORE_AGENT_ORDER` in `agent-priority-order.ts` controls object key insertion order
+2. `agent-key-remapper.ts` injects ZWSP-prefixed runtime names into the `name` field for OpenCode's `localeCompare` sort
 
-### Why This Matters
+### Why Two Mechanisms
+
+OpenCode's `Agent.list()` sorts agents by `name` field via `localeCompare`. Object key order alone is not enough. The `name` field carries ZWSP prefixes (1-4 chars) so core agents sort before alphabetically-named agents.
+
+ZWSP is intentionally used in the `name` field only. It MUST NOT appear in:
+- Object keys (used as HTTP header values, causes RFC 7230 violations)
+- Display names returned by `getAgentDisplayName()`
+- Config keys
+
+### History
 
 Agent ordering has caused 15+ commits, 8+ PRs, and multiple reverts due to:
-1. ZWSP (zero-width space) prefix attempts that leaked into HTTP headers
+1. Early ZWSP attempts that leaked into HTTP headers via object keys
 2. Object.entries() iteration order depending on merge sequence
 3. Multiple code paths assembling agents differently
 
-### The Permanent Fix
-
-1. `CANONICAL_CORE_AGENT_ORDER` is the ONLY source of truth
-2. Core agents are always first in canonical order
-3. Non-core agents are sorted alphabetically for determinism
-4. 100-permutation regression tests ensure order stability
-
 ### Forbidden Patterns
 
 DO NOT introduce:
-- ZWSP or Unicode sort prefixes
+- ZWSP in object keys or display names (only allowed in `name` field via `getAgentRuntimeName()`)
 - Runtime sort shims or comparators
 - Alternative ordering constants
 - Object.entries() order dependencies