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Phase 7h: sqlrite-mcp — MCP server adapter (incl. 7g.8 ask tool) (#73)

* Phase 7h: sqlrite-mcp — MCP server adapter (incl. 7g.8 ask tool)

Adds a new workspace-member crate `sqlrite-mcp` with a single
`[[bin]]` target that exposes a SQLRite database as a Model Context
Protocol (MCP) server over stdio. LLM agents (Claude Code, Cursor,
Codex, mcp-inspector) spawn it as a subprocess and get seven tools
for driving the database without any custom integration:

- list_tables    — discover what's in the DB
- describe_table — column metadata + row count for one table
- query          — run a SELECT, return rows as JSON
- execute        — DDL / DML / transactions (hidden under --read-only)
- schema_dump    — full CREATE TABLE script (same dump `ask` uses)
- vector_search  — k-NN lookup against a VECTOR column
                   (uses HNSW index if present, brute-force otherwise)
- ask            — natural-language → SQL via sqlrite-ask
                   (Phase 7g.8 — gated behind default-on `ask` feature)

Hand-rolled JSON-RPC 2.0 over line-delimited JSON on stdio. ~1100
LOC for the whole binary; no tokio, no async runtime, no third-party
MCP framework — same dep-frugal theme as sqlrite-ask's hand-rolled
JSON over ureq. Sync, single-client, strictly serial dispatch.

Notable implementation details:

- **stdio_redirect.rs**: the engine's `process_command` calls
  `print!`/`println!` for REPL-convenience output (CREATE schema
  dump, INSERT row dump, SELECT result table) — those would corrupt
  the MCP protocol channel. Solved with a `dup2(2, 1)` dance at
  startup that redirects fd 1 to fd 2; JSON-RPC responses go through
  a saved-off duplicate of the original fd 1. The same pollution
  affects the existing SDKs but isn't visible there because their
  stdout doesn't matter; fixing it in the engine is a future cleanup.

- **Read-only mode**: `--read-only` opens the DB with a shared lock
  via `Connection::open_read_only` AND hides `execute` from
  `tools/lis... (continued)

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