Add configurable discovery cache directory

[sarvesh1327]

Context

Closes #76.

Changes

• Add a global --cache-dir flag and top-level [tool.tryke] cache_dir config option.
• Resolve cache directory precedence as CLI override > config > existing <project-root>/.tryke/cache default.
• Thread the resolved cache directory through test discovery, changed/changed-first, watch, server, and graph paths.
• Store custom cache data as <cache-dir>/discovery-v1.bin and use narrow custom-cache .gitignore entries so user-provided directories are not broadly ignored.
• Document the new config/CLI surface and add regression coverage.

Test plan

• cargo fmt --all --check
• cargo test -p tryke_config cache_dir -- --nocapture
• cargo test -p tryke cli::tests:: -- --nocapture
• cargo test -p tryke_discovery custom_cache_dir -- --nocapture
• cargo test -p tryke_discovery configured_cache_dir -- --nocapture
• cargo test -p tryke_discovery cache::tests::default_cache_writes_broad_gitignore_in_state_dir -- --nocapture
• cargo test -p tryke_discovery cache::tests::custom_cache_dir_writes_narrow_gitignore -- --nocapture
• uv run cargo test --workspace --all-features
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• uvx prek run -a

[Copilot]

Pull request overview

This PR adds a configurable location for Tryke’s persistent discovery cache, exposed via a new global --cache-dir CLI flag and [tool.tryke] cache_dir in pyproject.toml, and threads the resolved cache directory through discovery-driven flows (test, watch, server, graph).

Changes:

• Add global --cache-dir flag and cache_dir config option with precedence CLI > config > default <project-root>/.tryke/cache.
• Plumb cache directory overrides through discovery entry points (test discovery variants, watch, server, graph).
• Update cache persistence to support custom cache directories and write narrower .gitignore rules for user-provided cache dirs; add regression tests and docs.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
docs/reference/cli.md	Documents --cache-dir for top-level and relevant subcommands.
docs/guides/configuration.md	Documents [tool.tryke] cache_dir and CLI override behavior.
crates/tryke/src/cli.rs	Adds global --cache-dir flag and parsing tests.
crates/tryke/src/main.rs	Resolves cache dir and threads it through test/watch/server/graph flows.
crates/tryke/src/discovery.rs	Adds *_with_cache_dir discovery functions and preserves existing APIs via wrappers.
crates/tryke/src/watch.rs	Extends watch entrypoint to accept an optional cache dir override.
crates/tryke/src/graph.rs	Adds cache-dir-aware graph/fixture graph functions and wrapper compatibility.
crates/tryke_server/src/server.rs	Adds server support for passing an optional cache dir into discovery.
crates/tryke_discovery/src/discoverer.rs	Adds cache-dir-aware constructors and routes cache loading accordingly; adds tests.
crates/tryke_discovery/src/cache.rs	Adds cache-dir-based loading and best-effort .gitignore writing behavior; adds tests.
crates/tryke_config/src/lib.rs	Adds cache_dir to config, resolves relative paths, and provides precedence resolver + tests.

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[thejchap]

@sarvesh1327 thank you!