Re-emit chat config after plugin resolution

[rschmukler]

Plugin-provided agents (and anything from installed plugins) can be missing from
the editor's agent picker, even though the server has loaded them correctly.
Symptom: the editor shows only built-in agents (code, plan) plus any
project-local .eca/agents/* agents, while plugin MCP servers still show up as
available. Querying the server's /api/v1/session shows the full agent list, so
the agents are loaded — the editor just never gets told about them.

Root cause

At initialized, two independent futures run:

1. Model sync — on completion it sends a config/updated notification that
   carries the agent list (:agents (primary-agent-names …)).
2. Plugin resolution — reset!s plugin-components*, which is what makes
   plugin agents visible to config/all.

These race. When model sync finishes before plugin resolution, it advertises
the pre-plugin agent list, and nothing re-emits it afterward — so plugin agents
never reach the editor. This is reliably hit with providers configured with
"fetchModels": false, since model sync then returns almost instantly and
always wins the race. (It also explains selectAgent falling back to code
when the configured defaultAgent is a plugin agent.)

Fix

• Extract the config/updated payload building into notify-chat-config-state!.
• The model-sync callback calls it (unchanged behavior — it already re-reads
  config/all).
• Call it again right after config/deliver-plugins-resolved! in the plugin
  future.

notify-fields-changed-only! diffs against the session-level mirror, so the
extra call only emits the newly-resolved agents and is a no-op when plugins won
the race. Either ordering now converges on the full agent list.

Testing

• clj-kondo clean (0 errors / 0 warnings).

• Namespace compiles.

• eca.handlers-test (12 tests / 78 assertions) and
  eca.features.plugins-test + eca.config-test + eca.server-test
  (32 tests / 260 assertions) pass.

• I added a entry in changelog under unreleased section.

• This is not an AI slop.

[ericdallo]

thank you!