docs: add extended ROADMAP and pre-merge checklist (#21)

ROADMAP.md expands the README's brief roadmap section into the full
phased plan: current shipped state, design principles, the Rust CLI
port (usable at each stage), the event-driven monitoring agent layer,
the longer-horizon observability toolkit items, open design questions,
and explicit non-goals.

PRE-MERGE-CHECKLIST.md defines what 'done' means before a merge, with
documentation currency as the headline section: a table mapping each
kind of change to the doc surfaces it touches, a skill-lockstep rule,
and an explicit requirement that AI agents state which docs they
verified and how. Also covers tests/CI gates, packaging invariants,
public-repo hygiene, and security posture.

README's Roadmap section now links to ROADMAP.md; CONTRIBUTING's
guidelines now reference the checklist.

Co-authored-by: Agent for Leonard Janke <leonard.janke.agent@proton.me>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>