feat(cli): wire --resume / --continue / --fork-session to resume sessions

[oratis]

The --resume / --continue / --fork-session flags were parsed but ignored — the REPL always started a fresh session (🔄 "parsed only" in BEHAVIOR_PARITY.md). This wires them through startRepl → a new resolveSession():

• --resume <id> / -r <id> — resume that session, seeding its stored history into the agent context.
• --resume / -r (no id) — interactive picker listing recent sessions (most-recent-first).
• --continue / -c — resume the most-recently-updated session in the current cwd.
• --fork-session — resume into a new session seeded with a copy of the source history, leaving the original intact.

Unknown id / no-session-in-cwd fall back to a fresh session with a notice. -r / -c short aliases added for Claude Code parity.

Tests

• repl-resume.test.ts — 6 cases over resolveSession (fresh · resume-by-id · continue-most-recent-in-cwd · continue-none · fork-copies-and-preserves-source · unknown-id-fallback), pure over a SessionManager rooted at a temp dir.
• Smoke-tested end-to-end against a throwaway $HOME: --continue resumed the newest in cwd; the picker listed + resumed the chosen session (validates the dual-readline); --fork-session created a new id (session count 2→3) leaving the source intact.
• Full CLI suite: 124 passing.

Flips the --resume/--continue/--fork-session row in BEHAVIOR_PARITY.md 🔄 → ✅.

Follow-up (not in scope)

• In-REPL /resume mid-session switching (still list-only).
• Replaying the prior transcript to the screen on resume (currently a one-line "Resumed N messages" notice; the model already gets full context).

🤖 Generated with Claude Code