fix(agents): keep object keys clean by separating list display from runtime name

Discovered via post-implementation review (Oracle goal verification): the
prior commit's prefix swap (ZWSP -> ASCII spaces) inherited a pre-existing
architectural bug from the ZWSP era. `getAgentListDisplayName()` was an
alias for `getAgentRuntimeName()`, which meant every callsite that used
the "list display" name as an OBJECT KEY (config.agent keys, lookup keys,
HTTP-header-bound paths) ended up carrying the sort prefix.

This worked silently with ZWSP because zero-width characters are visually
invisible. With ASCII space prefixes, the same bug becomes user-visible
and violates the explicit RFC 7230 constraint documented in AGENTS.md:
"ZWSP MUST NOT appear in object keys (used as HTTP header values)."

Fix: separate the two concepts that were conflated.

- `getAgentListDisplayName(key)` now returns the CLEAN display name
  (alias of `getAgentDisplayName`). Used for object keys, config keys,
  and any path where the name will be sent over HTTP.
- `getAgentRuntimeName(key)` keeps its prefixed return value. Used ONLY
  for the `.name` field that OpenCode reads for `localeCompare` sort.

`agent-key-remapper.ts` was already correct: it uses `getAgentRuntimeName`
for the `.name` field. The bug was that `getAgentListDisplayName` (used
as the object key) also returned the prefix.

Test updates:
- agent-display-names.test.ts splits the assertions: getAgentListDisplayName
  asserts clean names, new getAgentRuntimeName describe asserts prefixes
- All other tests using getAgentListDisplayName as an expected object key
  continue to pass because they always wanted clean names

Verification:
- bun test: 5769 pass / 10 pre-existing failures (unchanged)
- bun run typecheck: clean
- Manual: agent-key-remapper output keys verified RFC 7230 safe (no
  leading whitespace, no ZWSP); name fields preserve descending-space
  prefix for canonical core agent ordering

src/shared/agent-display-names.ts

@@ -50,31 +50,20 @@ export function getAgentRuntimeName(configKey: string): string {
   return prefix ? `${prefix}${displayName}` : displayName
 }
 
-/**
- * Get display name for an agent config key.
- * Uses case-insensitive lookup for backward compatibility.
- * Returns original key if not found.
- */
 export function getAgentDisplayName(configKey: string): string {
-  // Try exact match first
   const exactMatch = AGENT_DISPLAY_NAMES[configKey]
   if (exactMatch !== undefined) return exactMatch
-  
-  // Fall back to case-insensitive search
+
   const lowerKey = configKey.toLowerCase()
   for (const [k, v] of Object.entries(AGENT_DISPLAY_NAMES)) {
     if (k.toLowerCase() === lowerKey) return v
   }
-  
-  // Unknown agent: return original key
+
   return configKey
 }
 
-/**
- * Runtime-facing agent name used for OpenCode list ordering.
- */
 export function getAgentListDisplayName(configKey: string): string {
-  return getAgentRuntimeName(configKey)
+  return getAgentDisplayName(configKey)
 }
 
 const REVERSE_DISPLAY_NAMES: Record<string, string> = Object.fromEntries(