Sessions: canonical v1 writer and cross-process ownership

[oratis]

Summary

• define one canonical <id>.v1.jsonl schema for core and desktop writes
• normalize legacy core/desktop sessions into an atomic sidecar stream on first write without changing legacy bytes
• use the same create-new <id>.writer.lock ownership contract in TypeScript and Rust
• prefer canonical reads, deduplicate listings, and preserve legacy metadata during Rust normalization
• make explicit archive/delete operate on both generations
• document format, recovery, corruption, and stale-lock policy

Validation

• pnpm docs:check
• pnpm lint (3 pre-existing warnings, 0 errors)
• pnpm format:check
• pnpm typecheck
• pnpm test
• pnpm build
• cargo check --manifest-path apps/desktop/src-tauri/Cargo.toml
• cargo test --manifest-path apps/desktop/src-tauri/Cargo.toml (32 passed)

Stack

Depends on #184 through #180. This completes the canonical writer/ownership slice; stale-lock recovery moves to the future single-owner app-server rather than unsafe client lock stealing.