ADR relationship graph, public readiness docs, and harness-parity CI - #3

Regenerates docs/decisions/README.md and relationships.mmd/svg to include
ADR-0011, and updates handoff.md with verification results (395 tests,
11 ADRs validated, 6 graph edges) ahead of PR follow-up.

ADR-0011 brought the count to 11; the navigation-and-scale note still
referenced 10.

Baseline open-source hygiene ahead of the repository going public:
a Code of Conduct with report/enforcement contact matching SECURITY.md's
convention, and bug report / feature request issue templates scoped to
this project's commands and existing roadmap/improvements tracking.

Re-ran the same validate/install/list/script-layer checks documented for
the Codex and Gemini CLI adapters, this time against the agy CLI, which is
in fact available in this environment (contrary to the prior README note).
All three script commands (preflight, init, validate) returned "ok": true
from the agy-installed snapshot in an isolated HOME, closing the one real
Harness parity gap: Antigravity was the only adapter with no end-to-end
verification on record.

Existing tests only check that adapter manifests are schema-valid JSON;
nothing catches an upstream CLI change or a manifest edit that breaks real
installation. This job installs the real Codex CLI (0.151.0) and Gemini
CLI (0.46.0) -- the exact versions manually verified in adapters/codex and
adapters/gemini-cli's READMEs -- and drives their own plugin/extension
commands (marketplace add, install, list) against this repo, then runs
preflight/init/validate from the installed snapshot and asserts "ok":
true on each. Every step was dry-run locally against those exact CLI
versions before landing this.

Antigravity CLI (agy) has no npm/package-registry distribution, so it
stays a manually verified adapter only per adapters/antigravity/README.md.

project-roadmap.md's Harness parity section now reflects that Codex/Gemini
CLI install verification is automated in CI; the remaining gaps are
extending coverage past preflight/init/validate and automating Antigravity
once it has an installable distribution. handoff.md rewritten to describe
what this session actually did instead of the stale ADR-0011-in-progress
state, and flags that harness-parity has only run locally, never on a real
GitHub Actions runner yet.

Codex CLI and Gemini CLI now both have stable hook extension points
(hooks.json SessionStart/UserPromptSubmit; gemini hooks migrate), so the
roadmap item's precondition is technically satisfied. Decided not to
pursue it anyway: ADR Toolkit doesn't use a hook even on Claude Code today
(pure skill auto-discovery), an always-on session hook cuts against the
project's own minimal-interruption principle, and the plausible use cases
are already covered by deliberately invoking discover/check. Recorded so
the same discussion doesn't have to happen again from scratch.