Summary

• FileExecutor::new now expands ~ in allowed_paths using dirs::home_dir() before canonicalization
• Previously, tilde-prefixed paths silently blocked all file access (sandbox violation on every tool call)
• Fix is contained entirely in FileExecutor::new — no call-site changes required

Root Cause

Path::canonicalize("~/...") returns Err because tilde is not a shell metacharacter in Rust filesystem APIs. The unwrap_or(p) fallback retained the literal ~/ path, so starts_with never matched real absolute paths.

Changes

• crates/zeph-tools/src/file.rs: add expand_tilde() helper, apply in FileExecutor::new
• crates/zeph-tools/Cargo.toml: add dirs workspace dep
• 4 new unit tests: tilde_path_is_expanded, absolute_path_unchanged, tilde_only_expands_to_home, empty_allowed_paths_uses_cwd

Test Plan

• cargo +nightly fmt --check passes
• cargo clippy --workspace --features full -- -D warnings passes (0 warnings)
• cargo nextest run --workspace --lib --bins passes (5919/5919)
• New tests: tilde_path_is_expanded, absolute_path_unchanged, tilde_only_expands_to_home, empty_allowed_paths_uses_cwd

Closes #2115