ToolSearch preload threshold

Problem

Deferred tools (shouldDefer=true) are unconditionally hidden behind ToolSearch: every MCP tool (hardcoded in DiscoveredMCPTool) plus a set of bundled built-ins (web_search, web_fetch, cron, monitor, worktree, …). Deferral saves prompt tokens when the deferred set is large, but it is not free: every mid-session reveal rewrites the function-declaration list, which sits at the front of the tools→system→messages prefix, so a single ToolSearch load invalidates the entire prompt KV cache. For a small deferred set the deferral saves little and the cache damage plus the extra ToolSearch round-trip make it a net loss.

Claude Code models this tradeoff with ENABLE_TOOL_SEARCH=auto / auto:N: "tools load upfront if they fit within 10% of the context window, deferred otherwise" (code.claude.com/docs/en/agent-sdk/tool-search). This change adds the equivalent gate.

Design

New setting tools.toolSearch.threshold (number, percent, default 10).

At session start (GeminiClient.startChat, before the deferred-tools reminder is resolved), when ToolSearch is registered and the threshold is > 0:

• Estimate the combined token footprint of every deferred tool schema — bundled built-ins and MCP alike (JSON.stringify(tool.schema).length / CHARS_PER_TOKEN).
• If the total fits within threshold% of the context window (contentGeneratorConfig.contextWindowSize, falling back to tokenLimit(model)), reveal them all via the existing revealDeferredTool mechanism. All-or-nothing — a partial reveal would leave an arbitrary subset behind ToolSearch, and any tool left deferred can still bust the cache on first use.
• Otherwise everything stays deferred (previous behavior). threshold: 0 restores the old behavior unconditionally.

Preloaded tools therefore land in the initial declaration list, are filtered out of the startup deferred-tools reminder, and the declaration list stays stable for the whole session.

Decisions

• Session start only, never setTools(). Revealing a tool the startup reminder already announced would make queueAddedMcpToolsReminder flag it as "removed", and a mid-session declaration change busts the very cache the preload exists to protect. Tools from servers that connect later stay deferred (announced via the added-tools reminder, reachable through ToolSearch) until the next session start. /clear clears the revealed set and re-runs the decision.
• One budget over the whole deferred set, bundled included. Claude Code's auto threshold covers MCP/SDK tools only (its built-ins are managed separately), but it can afford that split: deferred tools are stripped from the prompt prefix before the cache key is computed, and a discovered tool's definition is expanded inline via a tool_reference block — "The prefix is untouched, so prompt caching is preserved" (platform.claude.com/docs/en/agents-and-tools/tool-use/tool-search-tool). Here every reveal — bundled or MCP — goes through setTools() and rewrites the declaration list. Excluding the ~14 bundled deferred tools (web_search, web_fetch, …) would leave the prefix one common tool-load away from a full cache bust, forfeiting exactly the stability the preload buys. When the union exceeds the budget everything stays deferred, which matches the pre-threshold baseline for bundled tools.
• Threshold defaults to 10 (auto-mode on), unlike Claude Code's default. Claude Code's unset default keeps MCP tools always deferred and makes auto opt-in — affordable there because a deferred tool's first use costs no cache invalidation. Here it costs a full prefix rebuild, so the auto-style gate is on by default; threshold: 0 reproduces Claude Code's always-defer default.
• Already-revealed tools count toward the budget so repeated session starts (compression also passes through startChat) cannot ratchet the revealed set past the budget as servers come and go.
• No preload when ToolSearch is unavailable — the existing eager-reveal branch in resolveDeferredToolsForReminder already exposes everything.