feat: add transactional Pickforge init

[ElbertePlinio]

What changed

• adds experimental pickforge init [--dry-run] [--json] planning and apply contracts
• writes deterministic external project receipts under the Pickforge state root
• adds transactional, idempotent Claude Code, Codex, and Pi adapter foundations with retained backups and rollback reporting
• preserves foreign config, refuses unsafe targets, and recognizes owned interruption artifacts for rerun convergence
• keeps the base integration pack empty; real Flutter MCP/workflow content remains PR 3

Validation

• cargo fmt --all --check
• cargo clippy --workspace --all-targets --locked -- -D warnings
• cargo test --workspace --locked — 56 Linux tests
• Windows MSVC all-target cargo check and cargo clippy -D warnings
• Bun 1.3.12 frozen install, typecheck, lint, and build
• OSV Scanner 2.3.8 — no unfiltered advisories
• serious-backend local review plus targeted fix verification — clean

Not tested / known boundaries

• Windows-native tests run in PR CI; macOS remains deferred
• no durable crash journal: atomic per-file writes, in-process rollback, and rerun convergence are the intentional boundary
• no Flutter pack content, MCP migration, packaging, release exposure, or product rename in this slice

Risk class: serious-backend

Stacked on #65. Completes PR 2 of the six-PR alpha plan; PR 3 adds the real Flutter integration pack.

Refs #64