fix: inject deferred MCP tools into system prompt and lift selectSearch cap

[OliverZou]

Summary

Two issues left the agent blind to available MCP tools:

1. The system prompt had no visibility into which MCP tools exist. The <available-deferred-tools> block was only surfaced inside tool_search responses, so the agent could not discover MCP tools without first knowing to call tool_search — a discoverability deadlock.

2. tool_search capped exact-name (select:) queries at maxSearchResults=5, silently dropping the rest even when the user explicitly named them.

Fixes:

• Inject <available-deferred-tools> into every turn via a dynamic system block so the agent always knows what MCP tools are available.
• Cap the block at 4000 characters (newline-boundary truncation) with an omission notice so many tools don't blow up the prompt.
• Remove the maxSearchResults cap from selectSearch so select: queries return every requested tool, not just the first five.
• Show all tool names (not just the first five) in tool_search no-match hints, aligning with the new system-prompt visibility.
• Add tests for the new render path, selectSearch cap removal, truncation behavior, and updated hint format.

Validation

• Local test-windows — 7/7 packages pass
• Focused tests — all relevant suites pass
• Linux CI — deferred to GitHub Actions

User-visible impact

• System prompt: each turn now includes an <available-deferred-tools> block listing all connected MCP tools.
• tool_search no-match hints: now show all tool names with a total count, no more silent truncation to five.
• tool_search select: queries: no longer capped at 5 results.

Breaking changes

None.

Notes

• The 4000-character cap prevents system-prompt blowup when many MCP tools are registered.
• Test coverage added in deferred_test.go, mcp_runtime_test.go, and catalog_mcp_test.go.