[207] feat: selective build cache sharing between worktrees

[erishforG]

Summary

Add [worktree] config section so parsec start can reuse build artifacts (e.g. target/, node_modules/, .venv/) from the main repo into new worktrees, instead of forcing a cold rebuild.

[worktree]
shared_cache    = ["target", "node_modules", ".venv"]
cache_strategy  = "symlink"   # "symlink" | "copy"

For each entry, the source <repo_root>/<entry> is shared into the new worktree using the chosen strategy. Missing sources and pre-existing destinations are skipped. Sharing failures are logged but never fail the worktree itself. Default is an empty list, preserving prior behaviour.

Closes #207

Changes

• src/config/settings.rs — [worktree] section, CacheStrategy enum, deserialization.
• src/worktree/cache_share.rs (new) — share logic with skip-on-missing / skip-on-exists / traversal-guard / failure-as-warning semantics.
• src/worktree/manager.rs — invokes share_cache after git worktree add succeeds.
• schema/parsec-config.schema.json — schema entries.
• README.md, CHANGELOG.md — docs.
• tests/cli_tests.rs — 6 new integration tests.

Verification

Check	Result
cargo build --release	✅
cargo test	✅ 51 passed (11 unit + 40 integration; 6 new)
cargo clippy --all-targets -- -D warnings	✅ clean (also fixed a pre-existing bool_assert_comparison lint)
cargo fmt -- --check	✅ clean

Test plan

• CI green on develop target
• Manual: parsec start <ticket> with shared_cache = ["target"] and cache_strategy = "symlink" — verify symlink is created in the new worktree pointing at the parent's target/
• Manual: same with cache_strategy = "copy" — verify it's a real dir, not a symlink
• Manual: missing source dir in shared_cache → no error, just skipped (warning logged)