feat(tools): bump-engine + engine lock parity doctor check

[joeseverino]

One flow for moving the MCP fleet to a newer vault-engine, and a doctor gate for the invariant it maintains.

• lib/tools/engine.sh: engine_consumers + engine_lock_pin — the consumer list and uv.lock parse, defined once, shared by both faces
• tools bump-engine [--lock-only]: uv lock --upgrade-package + uv tool reinstall for severino-vault-mcp and severino-edu-mcp together
• tools doctor: 'engine lock parity' check — red when the two consumers pin different engine commits, warn (not fail) when the repos aren't present (CI)
• tests/engine-lock.bats: hermetic coverage (UV_BIN stub, MCP_HOME/EDU_MCP_HOME seams)

Backlog: task-engine-version-skew-guard-both-mcps-lock-the-same-vault-engi