feat: add durable debugging sessions

[shanejonas]

What

• Persist sessions and annotations in SQLite.
• Share navigation, filtering, fullscreen panels, and annotations between people and agents.
• Bundle the agent skill and document jsonrpc-debugger --skill.
• Release as v0.3.0.

Verification

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --locked
• cargo package --locked