feat: bootstrap pure Rust governance core

[zywkloo]

Stack

Draft stacked on #33 (feat/machine-protocol-v1). Rebase/retarget to main after #33 merges.

Summary

• add a Cargo workspace with a pure wtcraft-core crate
• model task stages, responsible roles, transition writers, session states, and session validation
• add deterministic reconciliation of task/session/Git facts into protocol alarms
• clarify the protocol distinction between a stage's responsible role and a transition's single writer
• document the Rust core boundary and contributor-friendly bootstrap backlog
• add zero-context contributor onboarding, a Rust glossary, workspace status, and a bounded first task
• add Rust fmt, test, and clippy gates to CI without replacing the Bash CLI

Suggested first contribution

A new Rust contributor can start with the session transition matrix tests described in docs/onboarding/rust-core-first-contribution.md. This is test-only and scoped to crates/wtcraft-core/src/session.rs.

Deliberate non-goals

• no Git or process execution
• no task/session file mutation
• no daemon or GUI integration
• no replacement of the installed wtcraft / wtc command

Testing

• cargo fmt --check
• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings
• bash tests/run_all.sh
• ./scripts/wtcraft verify .
• ./scripts/wtcraft check .