The TUI retry-allowance follow-up made tui_gateway.entry.wait_for_mcp_discovery
delegate to hermes_cli.mcp_startup unconditionally when no entry-local thread
exists. But server._make_agent already calls the startup wait directly for
dashboard /api/ws sessions, so every non-stdio agent build paid the bounded
wait twice (caught by test_make_agent_waits_for_shared_mcp_discovery). Gate
the retry-spawn AND the delegated wait on _mcp_discovery_enabled, which only
the stdio TUI arms in main().
Builds on fazerluga-creator's #66981 (cherry-picked as the previous two
commits). start_background_mcp_discovery()'s retry allowance only fires
when the function is CALLED again, but tui_gateway/entry.py main() calls
it exactly once at startup — so a first discovery run that connected
nothing still latched the stdio TUI MCP-less for the whole session.
Re-invoke the idempotent spawn from wait_for_mcp_discovery() (the
per-agent-build wait) when the process is MCP-enabled, gated on a flag
set in main() so non-MCP sessions never pay the MCP import on the wait
path. Adds regression tests for both the retry re-invocation and the
non-MCP skip.
Review follow-up: the stdio hermes --tui path spawned its own one-shot
discovery thread, so the retry-after-zero-connected semantics added to
start_background_mcp_discovery() did not cover it.
Spawn TUI discovery through the shared owner and make the entry-side
wait_for_mcp_discovery() fall through to the shared owner when no local
thread exists (mcp_discovery_in_flight/join_mcp_discovery already consult
both owners). Keeps the cheap no-mcp-servers config guard on the TUI path.
Adds regression tests for the entry-side wait delegation.
