Fresh Claude JSONL can be ignored when created before tailer snapshot #308

Problem

A fresh Claude pane can create its root JSONL transcript in the narrow window after the PTY process is spawned but before Agent Code / claude-code-headless has finished wiring the fresh-session directory watcher.

The old fresh-session tailer snapshots existing files and ignores them. If the new transcript lands before that snapshot, it is classified as pre-existing and ignored forever. Agent Code can then keep receiving proxy/semantic traffic while committed JSONL remains disconnected and providerSessionId never becomes durable.

Scope

This is the narrow attach race called out in #290. It does not claim to solve every disconnected-transcript case where Claude never writes the expected root JSONL at all.

Acceptance

• Agent Code captures a timestamp before spawning a fresh Claude PTY.
• claude-code-headless can treat JSONL files with mtime/ctime after that timestamp as fresh-session candidates.
• Normal resume behavior is unchanged.

Related: #290, #283.