import: share dir-resolution and timestamp parsing across importers

Factor the last cross-importer duplication into shared helpers:

• resolveDir (discover.go): the overridePath-or-GetSessionDir block every Discover repeated; each Discover now resolves the dir in one line.
• parseTimestamp (linesplit.go): the RFC3339-or-zero parse used by cursor, pi, copilot, claude, and codexLineTime.

With discoverSessionFiles, splitLineTurns, resolveDir, and parseTimestamp, all the agent-agnostic plumbing is now shared; what remains per importer is the genuinely format-specific part — prompt-line detection and which agent token/model parsing to call (per-turn import needs each prompt's native-space line offset, which no agent method exposes, so detection can't be reused from the agent layer). Pure refactor; behavior unchanged, verified against the importtest fixtures and existing tests.