feat(mcp): add write tools for accounts, assets, and transactions

[a-mandala]

Summary

• Extends the portfolio MCP server with 9 write tools alongside the existing read-only ones
• All write tools reuse the existing storage layer, so validation (negative positions, OPENING constraints, referential integrity) matches GraphQL mutations exactly

New tools

Tool	Purpose
create_account / delete_account	account management
create_asset / delete_asset	tracked-asset management
create_transaction	record BUY/SELL/OPENING
update_transaction	full replace of an existing transaction
delete_transaction	remove a transaction
create_transfer	cash between accounts, optional FX conversion
create_cash_movement	deposit/withdrawal

Notes

• Server instructions updated to advertise the new surface
• Tool-list test extended to the full 12-tool set
• No auth added (out of scope, per discussion) — MCP endpoint exposure unchanged

Validation

• cargo check clean
• cargo test: 258 passed, 0 failed (incl. updated tool-list test)
• cargo fmt applied, cargo clippy no new warnings