import: add importers for cursor, pi, factory, codex, copilot, gemini

Extends entire import beyond Claude Code to the other supported agents. PR #1527 built the agent-agnostic orchestration (discovery loop, idempotent per-turn IDs, redaction, checkpoint write) behind an Importer seam; this adds one self-contained Importer per agent and registers them, so each entire import <agent> subcommand surfaces automatically.

Each importer reuses its agent's existing transcript methods rather than re-encoding parsing:

• cursor: shared transcript.ExtractUserContent; no native token/model data
• pi: CalculateTokenUsage + ExtractModel (branch-aware), <ts>_<uuid> filenames
• factoryai-droid: subagent-aware CalculateTotalTokenUsage; model from the adjacent .settings.json
• codex: walks the global sessions tree, filters by session_meta cwd, uses the cumulative-usage delta for per-turn tokens
• copilot-cli: flat session-state store, filters by session.start cwd/gitRoot
• gemini: single JSON document, imported per-session (message-index offsets)

Codex and Copilot share a repoMatches helper (equal-or-descendant, symlink normalized) so their global/flat stores only import this repo's sessions. Imported checkpoints keep the merged Claude behavior: commit-less, Kind "imported", not rewindable, idempotent.