chore(workers): platform-aware default paths

[intendednull]

Why

Hardcoded /etc/willow/*.key + /var/lib/willow/storage.db in clap default_value flags. Linux-only. Bad on macOS/Windows dev. TD-06.

What

• Add default_replay_key() in crates/replay/src/main.rs. Use dirs::config_dir(). Fallback /etc/willow/replay.key.
• Add default_storage_key() + default_storage_db() in crates/storage/src/main.rs. Use dirs::config_dir() for key, dirs::data_dir() for db. Fallback to historical Linux paths.
• Wire via default_value_t = helper().display().to_string(). Behaviour unchanged when flag passed (Docker images set own paths).
• Unit tests assert tail = willow/<file>, parent dir name = willow, path absolute.

Scope guard

• Touch only clap default_value = "/..." sites.
• No typed-config crate (TD-11 follow-up).
• /tmp/test-worker.key struct-field default left alone (TD-13).
• Docker compose unchanged. Dockerfiles set paths via flags.

Verify

rg "default_value\s*=\s*\"/" crates --type rust   # zero matches
cargo test -p willow-replay -p willow-storage     # 3 new helper tests pass
cargo fmt --check && cargo clippy --workspace --all-targets -- -D warnings
cargo check --target wasm32-unknown-unknown ...   # wasm clean

Refs #324

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