Add clean cache command

[sarvesh1327]

Context

Closes #77.

Changes

• Add a tryke clean subcommand for removing the persistent discovery cache.
• Use the same project-root resolution as discovery for the default <project-root>/.tryke/cache path.
• Honor global --cache-dir / [tool.tryke] cache_dir; for custom cache dirs, remove only Tryke-owned cache files (discovery-v1.bin and discovery-v1.tmp) to avoid deleting unrelated user data.
• Add regression coverage for default cleanup, subdirectory project-root resolution, custom cache-dir safety, and CLI parsing.
• Regenerate the CLI reference.

Test plan

• cargo test -p tryke_discovery clean_ -- --nocapture
• cargo test -p tryke clean_ -- --nocapture
• cargo clippy -p tryke -p tryke_discovery --all-targets --all-features -- -D warnings
• Manual default/custom cache smoke checks with cargo run -p tryke -- clean
• cargo fmt --all --check
• cargo run -p tryke_dev --bin generate-cli-docs -- --check
• uv run cargo nextest run --workspace --all-features (687 passed; nextest reported 2 existing leaky tests)
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• uvx prek run -a