refactor(tools): drop Wuu progressive tool loading for a stable prefix

[LeoLin990405]

Summary

Implements the decision in #168: removes Wuu's provider-neutral wuu_tool_search progressive loading mode so the inference-request prefix stays stable, and keeps the local search executor that provider-native discovery depends on.

Changes

• Loading policy is now native or flat. resolveToolLoadingModeForProvider resolves to native when the provider and model support deferred discovery, and flat otherwise. ShouldFallbackToWuuToolSearchByDefault had no remaining caller and is deleted.
• Explicit native on an unsupported path degrades to flat, visibly. Previously it silently selected Wuu progressive loading. It now falls back to flat and prints a notice naming the provider, the model, and the mode actually in effect — the user asked for deferred tools and is not getting them. Deduped per provider+model over the process lifetime, because tool loading is re-resolved on every session build and every provider switch.
• The local search executor is untouched. internal/tools/tool_search.go and the discovery path stay: OpenAI and Anthropic native tool search still need Wuu to search the local catalog and return loadable schemas through tool_search_output / tool_reference. Every Native* runtime test still passes unchanged.

One deviation from the issue, worth a look

The scope says to remove the legacy tool_search alias and boolean migration path. Removing them outright makes any existing config that sets agent.tool_loading = "wuu_tool_search" fail Validate(), which turns a retired tuning knob into a hard startup error for someone who set it once and forgot.

This keeps those spellings parsing and resolves them to auto, with a one-time deprecation notice naming the replacement:

config	before	after
tool_loading: "wuu_tool_search"	Wuu progressive	auto + notice
tool_loading: "tool_search" (alias)	Wuu progressive	auto + notice
tool_search: true	Wuu progressive	auto
tool_search: false	flat	flat (unchanged)

auto rather than flat because the intent behind those values was "defer tools where you can", and auto still gives native discovery on providers that support it. The validation message no longer advertises the removed mode, and an unknown value still errors. Happy to make it a hard error instead if you would rather force the edit.

Test plan

• Added or updated unit tests for the change
• go test ./... passes
• cd desktop && npm test passes
• Manually verified the behavior in the desktop shell (if applicable)

Three existing runtime tests asserted the removed behavior and now assert the flat outcome, including that the flat surface stops shipping the deferred-catalog prompt and declares the formerly deferred tools directly. New tests cover: the fallback notice contents and its per-provider+model dedupe, the retired values resolving to auto, a retired value still passing Validate(), supported values staying silent, and the legacy boolean mapping. make check passes; gofmt clean.

Not verified in the desktop shell — this is core-side resolution with no renderer surface, and I could not launch Electron in this environment.

Risk and rollback

• Risk level: medium — changes the tool surface sent to providers. No protocol or persisted-state change; the desktop and CLI read the resolved mode but do not set it.
• Behaviour change to call out: anyone previously on the Wuu progressive path now gets a flat tool list, so their first request carries roughly 7.8 KB (~2.6K tokens by Wuu's estimator) more schema, in exchange for a prefix that stays cacheable.
• Rollback: revert the commit.

Linked issues / docs

Closes #168