Agent sessions: memory tool not provisioned — agent sessions have no access to the memory tool available in editor chat (surface toolset parity gap)

### Summary

Copilot **agent sessions** (the Agents surface / background agent sessions) ship a reduced, fixed core toolset compared to an interactive **editor chat** session in the same workspace. Most notably, the `memory` tool is not provisioned at all in agent sessions.

### Steps to Reproduce

1. In a workspace with an established `/memories/` store, open Copilot Chat in the **editor** (ask mode). Inspect the model's available tools → `memory`, `read_file`, `edit`, `run_in_terminal`, notebook tools, symbol tools, etc. are present (~30 built-ins). The `memory` tool is functional.
2. Start a **Copilot agent session** (Agents surface) on the same workspace and ask it to list its tools / read a memory file.
3. Observe: agent session exposes only the fixed core set (~17 tools). No `memory` tool; no per-MCP tools unless the server happens to start.

### Expected

Tool parity (or documented intentional deltas) between editor chat and agent sessions. At minimum, memory should be provisioned in agent sessions so they can read/write the same `/memories/` store — otherwise agent sessions are amnesiac relative to chat sessions on identical work.

### Actual

- `memory` tool absent from agent sessions entirely (deterministic, reproducible across sessions).
- Agent sessions run with a smaller fixed built-in set (~17 tools) vs editor chat (~30).

### Related but distinct

- #331666 (memory tool writes nothing on /memories/ virtual paths) — different: tool exists but is broken there; here the tool is not provisioned at all.
- #309245 (memory tool never provisioned for GPT-5.4) — model-specific routing; here the gap is **surface-specific** (agent sessions), reproduced on other models.

### Environment

- VS Code 1.136.0, macOS arm64
- Copilot Chat extension (current as of 2026-09-03)

Filed from a real usage comparison (not synthetic). Happy to provide session IDs / tool dumps if useful.