feat(mcp): add $history variable and structured response format (nushell#17132)

<img width="1582" height="686" alt="image"
src="https://github.com/user-attachments/assets/538326ec-7ec8-45c0-a59f-f170ea88f5f8"
/>


- Add `$history` variable that stores all previous command outputs
- Access by index: `$history.0`, `$history.1`, etc.
- Wrap all responses in structured record with:
  - `history_index`: 0-based index of result in history
  - `cwd`: current working directory after command
  - `output`: command output (possibly truncated)
- Truncate large outputs (>10k chars by default) and reference history
- Configurable threshold via `NU_MCP_OUTPUT_TRUNCATE` env var
- Remove moka caching (incompatible with history tracking)
- Update instructions.md with history documentation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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Co-authored-by: Claude Opus 4.5 <[email protected]>

Author: andrewgazelka

Cargo.lock

@@ -24,6 +24,7 @@ tracing-subscriber = { version = "0.3.19", features = ["env-filter"] }
 bytes = { workspace = true }
 indoc = "2.0.6"
 nuon = { path = "../nuon", version = "0.109.2" }
+chrono = { workspace = true }
 
 [features]
 default = []