fix(agents): remove ZWSP prefixes from config.agent keys (#3238)

Agent names in the config.agent object (which becomes the /agent API
response) contained invisible Zero-Width Space (U+200B) characters
baked in by getAgentListDisplayName(). These ZWSP prefixes were used
for TUI sort ordering, but they leaked into the public API surface.

Impact: any prompt_async consumer that discovered agent names via the
/agent endpoint and passed them back to prompt_async without manual
ZWSP stripping got silent message drops — the agent name didn't match.
hy-pony's feishu-bridge integration went dark after upgrading to 3.16.0
with no error, no warning, and no indication that invisible Unicode
characters in agent names were the cause.

Fix: switch all four callsites from getAgentListDisplayName() (which
prepends \u200B×N) to getAgentDisplayName() (clean names):

- agent-key-remapper.ts: config keys → display names (was the primary
  injection point)
- agent-priority-order.ts: CORE_AGENT_ORDER lookup (must agree with
  the keys emitted by the remapper)
- command-config-handler.ts: command agent field normalization
- tool-config-handler.ts: agent config lookup (simplified fallback
  chain since the primary lookup is now clean)

Sort ordering is preserved by:
1. JS object insertion order from reorderAgentsByPriority()
2. The injected `order` field (1-4) added by injectOrderField()

getAgentListDisplayName() is marked @deprecated with a link to #3238.
AGENT_LIST_SORT_PREFIXES and stripAgentListSortPrefix() are kept for
any internal callers that strip prefixes from legacy data.

Closes #3238