Advertise present, manage_goal, manage_tasks, and lsp conditionally

[TheGreatAxios]

Summary

• present is removed from the advertised core set entirely; it stays fully dispatchable and is discovered via tool_search — at 2,793 chars it was the second-largest schema on the wire for a feature most sessions never use.
• manage_goal is advertised only when the session was resumed with an already-active/paused/budget-limited goal (checked once via loadGoalState before the first inference call), not whether a goal exists at the current instant.
• lsp is advertised only when a language server was resolvable for the project at startup (src/agent/lsp-availability.ts mirrors @intx/tools-lsp's TypeScript server check: a resolvable tsserver plus a reachable typescript-language-server binary), not whether one currently responds.
• manage_tasks stays unconditionally advertised in both session modes. goalKickoffUserMessage (src/agent/goal.ts) instructs the model to call manage_goal then manage_tasks back to back whenever a goal starts — gating the second would trade one tool_search round trip for two, which is the exact failure shape task/search_agents were promoted into the core set to avoid.
• ask_operator is untouched.

All four conditions are resolved once per session, before the toolset is built, and threaded through as an explicit ToolAvailability value (hasGoalAtLaunch, languageServerAvailable) rather than re-derived mid-session, since the tools array is a provider cache prefix.

Verification

• bun run typecheck, bun run test (3957 pass), and bun run build all pass.
• New/updated tests in src/agent/tool-search.test.ts cover: present absent from the core set, manage_goal/lsp gated on availability, manage_tasks/ask_operator advertised unconditionally, and the built-in prefix staying byte-identical across repeated calls with the same session-start availability (simulating turns).
• New src/agent/lsp-availability.test.ts covers detectLanguageServerAvailable against a seeded temp project and against this repo's own checkout.
• Measured in-repo: an orchestrator session with none of the four gated tools applicable drops the advertised tool-schema JSON from 18,366 to 12,948 chars (~4,592 to ~3,237 tokens at the chars/4 lower bound) — a 5,418-char (~1,355-token) reduction, on top of removing present from every session regardless of gate state.

Closes CL-5588

Tradeoff: manage_goal is gated on whether a goal exists at session start, so
a goal set mid-session costs one tool_search round trip before the model can
call it. Most sessions neither start with nor acquire a goal, and the wire
array must stay fixed for the prompt cache, so this is the intended trade
rather than an oversight.

[linear-code]

CL-5588