v0.8.15: custom providers and model parameters under Advanced options

The settings dialog could pick a preset, a model and a key and nothing else.
Two things were missing: a provider the presets do not know, and what the
model is asked to do while it answers. Both live under "Advanced options"
now, next to the entry they belong to.

Custom providers (engine)
- cst_models_user_set_provider() / cst_models_user_drop_provider() write the
  engine's own user layer, <config>/cst-agent/models.json:
  {"providers": {"myllm": {"api_base": ..., "key_env": ...,
                           "default_model": ..., "models": [...]}}}
  and reload the registry, so the provider is at once in the provider list,
  in the wire models[] and in the CLI's /provider -- the file a hand-written
  override would go into is the same one, so the GUI and an editor agree.
  The name is checked (1-31 of letters, digits, '.', '_', '-'), a preset's
  name overrides that preset (a self-hosted endpoint, a proxy), everything
  else in the file (model rules) is kept, and a file that is not a JSON
  object is never overwritten. Dropping only removes the user's own entry.
- AgentCore::providers() gained "custom" (source == "user"), and
  addProvider() / removeProvider() drive the two calls above.

Model parameters (engine + wire)
- cst_agent_config_t and cst_llm_request_t carry temperature, top_p and
  max_tokens. 0 means the field is not sent at all -- an absent field is the
  provider's own default, which "temperature": 0 is not (deterministic
  decoding is a real choice), so cst_llm_build_body() only adds what was set.
  tests/test_llm_body.c:test_gen_params pins both halves.
- settings.json keeps them per provider ("params": {...}) via
  cst_settings_remember_params(); cst_settings_remember() carries them along
  with the provider choice, and cst_agent_create() reads them, so a CLI or
  TUI run sends what the window was set to. An all-zero config (the CLI)
  leaves what the GUI saved alone.
- AgentCore::genParams() / setGenParams(), and useProvider() applies both the
  remembered parameters of the provider it lands on and a base URL that was
  edited in the dialog.

The dialog
- Rewritten around one scrolling body with a fixed footer: a long model list
  used to push the key field and the buttons out of the dialog. The model
  chips are filtered (a filter field appears past twelve) and capped at 40
  with "... +N more", which is what a router with hundreds of models needs.
- "Advanced options" folds out the provider entry (name, base URL, key
  variable, default model, models; Save provider / Remove from models.json)
  and the model parameters (temperature, top-p, max tokens; empty = not
  sent). The fields follow the picked provider and are what gets saved, so a
  hand-typed edit is never lost to a stale property.

Verified: --uitest 67/67 (15 new checks: parameters round-trip through the
engine, a provider added through the form and removed again, a bad name
refused, a preset not removable), --selftest, ctest cst-app-selftest, and
make check with the new body test.

Version 0.8.15.

mode = coding

app/README.md

@@ -148,6 +148,43 @@ into the field reaches the engine when "Use this" is clicked, the note says so,
 and Close puts the dialog away. The run points `XDG_CONFIG_HOME` at its scratch
 directory, so settings and keys of the real user are never rewritten.
 
+## Custom providers, model parameters
+
+The dialog's **Advanced options** covers the two things the presets cannot:
+a provider of your own, and what the model should do.
+
+- A custom provider is an entry the engine keeps in
+  `<config>/cst-agent/models.json` (layer 1 of the model registry, see
+  `include/cst_models.h`) — the same file a hand-written override would go
+  into, so the GUI and a text editor are interchangeable:
+
+  ```json
+  { "providers": { "myllm": { "api_base": "https://api.example.com/v1",
+                              "key_env": "MYLLM_API_KEY",
+                              "default_model": "m1",
+                              "models": ["m1", "m2"] } } }
+  ```
+
+  `AgentCore::addProvider()` / `removeProvider()` write it
+  (`cst_models_user_set_provider()` / `cst_models_user_drop_provider()`) and
+  reload the registry, so the provider is in the list, in `models[]` on the
+  wire and in `/provider` of the CLI at once. A name a preset uses overrides
+  that preset (a self-hosted endpoint, a proxy); the other content of the file
+  (model rules) is kept, and a file that is not a JSON object is never
+  overwritten. The key is still a key: it is verified against `GET /models` of
+  the new base URL before it is saved, and stored per provider name as before.
+- The generation parameters — **temperature**, **top-p**, **max tokens** — are
+  remembered per provider in `settings.json` (`"params": {...}`,
+  `cst_settings_remember_params()`), and applied to the engine's config by
+  `AgentCore::setGenParams()`, `AgentCore::useProvider()` and
+  `cst_agent_create()` (so a CLI or TUI run sends what the window was set to).
+  An **empty field is not sent at all** — that is not the same as sending 0,
+  which is a real temperature; `cst_llm_build_body()` only adds a field that
+  was set (`tests/test_llm_body.c:test_gen_params`).
+- A long model list (a router lists hundreds) is filtered and capped at 40
+  chips with "… +N more", and the whole dialog body scrolls: the buttons stay
+  put and nothing can spill out of the frame.
+
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