passdown v0.4.0

Added

• passdown-pickup skill: reads the latest handoff log and plan state,
  verifies them against the working tree, and briefs the next session —
  closing the shift-handover loop.
• Machine-readable YAML frontmatter (status, branch, agent, plan) at
  the top of every handoff log, plus a defined agent-identity convention.
• scripts/doctor.sh: reports install-channel hygiene per host — dual
  plugin/direct installs and direct installs that drifted from the checkout —
  with a regression suite wired into CI.

Changed

• passdown-dispatch now materializes routing decisions as [dispatch: ...]
  tags in the plan file and records a Dispatched: outcome line under each
  task executed off the main session.

Fixed

• Restore the 0.1.0 changelog entries and version link references dropped by
  the v0.3.0 release commit.
• The documentation contract tests now require a link reference for every
  CHANGELOG version heading, and check-version.sh requires a CHANGELOG
  section for the current version.