Fix: orchestrate/preflight.py behind-main check false-positives on feature branches + bare-mode silent death under set -e

Problem

Two preflight defects surfaced during #550's pod launch (2026-06-10):

1. Behind-main false positive on feature branches. orchestrate/preflight.py checks Local is N commit(s) behind origin/main — a pod checked out on a feature branch (e.g. issue-550, based on issue-538) trips this even when the branch is the intended run-of-record code. Workaround used: merge origin/main tip + revert content to clear the gate (history hack, tree unchanged).

2. Bare (non---json) preflight dies silently. The failure path logs through a handler-less logger, so under set -e the calling pipeline dies with ZERO output, frozen at the phase banner. Violates fail-loud.

Fix

• Make the behind-main check branch-aware: when HEAD is a non-main branch that tracks an origin/issue-* ref, check behind-ness against THAT ref (or skip with a logged note), not origin/main.
• Ensure bare-mode failures print to stderr (basicConfig or explicit handler) so set -e deaths are attributable.

Acceptance criteria

1. Preflight on a pod checked out on issue-<N> (up-to-date with its own origin ref) passes the git check.
2. A failing bare-mode preflight prints the failing check to stderr before exiting non-zero.
3. Existing main-branch behavior unchanged.

Reference: experimenter agent memory feedback_preflight_feature_branch_false_positive.md; incident task #550.