perf: dedupe TOML parsing + precompute env-name matcher (PR #460)

Address Copilot review: configure() now parses pyproject.toml and hatch.toml once each per workspace via read_workspace_hatch_sections(), and the env-name allowlist is precomputed into EnvNameMatcher so the try_from() hot path avoids per-call to_lowercase()/format!() allocations.

Author: karthiknadig