Add worktree-safe Compass graph bootstrap

[forhappy]

Summary

• add an idempotent compass ensure command that discovers the active Git worktree root and reports initialized, updated, or current
• keep mutable graph output isolated per linked worktree while reusing the existing incremental atomic update pipeline
• update installed assistant guidance to run compass ensure at session entry and after switching worktrees
• document the lifecycle and add an end-to-end linked-worktree regression test

Compatibility

This is an additive CLI and capability change. It does not change graph schemas or storage formats.

Verification

• cargo fmt --all -- --check
• cargo clippy --workspace --lib --bins --locked -- -D warnings
• cargo test --workspace --lib --bins --locked
• cargo clippy -p compass-cli --all-targets --all-features --locked -- -D warnings
• targeted Compass CLI help, capability, installer, and linked-worktree tests
• cargo test -p compass-cli --test compass_product --locked
• sh scripts/check_product_boundary.sh