fix(opencode): detect fresh and rotated sessions

[aaronflorey]

Summary

• keep fresh OpenCode launches unbound until a same-project session appears during the current startup window so agent-deck does not reopen an older conversation just because that directory has many historical sessions
• allow an already-bound OpenCode pane to rotate to a newer same-project session when there was recent local pane activity, which lets in-pane /new pick up the new session without stealing a sibling tab's binding
• add regression coverage for both bootstrap detection and recent-activity rotation, plus a debug log that explains when a rebind was triggered by recent local activity

Testing

• go test ./internal/session -run OpenCode
• go test ./internal/session (still has 2 pre-existing failures on this branch and on plain upstream/main: TestLifecycle_StoppedRestartedRunningError and TestStatusCycle_ShellSessionWithCommand)

Docs

• no documentation changes needed