docs: add CLI vs top-20 OSS competitive analysis and gap plans - #299

Summary

Adds a verified competitive analysis of graycode-cli vs the top-20 OSS CLIs (10 AI coding CLIs + 6 dev CLIs + 4 terminals incl. herdr multiplexer) plus five scoped gap plans. Builds on docs/plans/toolbench-comparison-vs-top20.md instead of duplicating it. All graycode claims re-verified from source (tool registry, sandbox fail-closed, share deeplink, custom providers, unwired backends).

Changes

• docs: new docs/COMPETITIVE.md (methodology, top-20 tables, 10 deep dimensions, wins/loses, gap index)
• plans: competitive-gap-01 docker onboarding (docs/UX only, fail-closed preserved)
• plans: competitive-gap-02 share links + multi-session visibility (local-first)
• plans: competitive-gap-03 Kitty graphics protocol (scopes pi-adoption-plan, no GPL copy)
• plans: competitive-gap-04 published benchmarks (docs from existing make bench/infra)
• plans: competitive-gap-05 default backend wiring via router facade (env-gated, boundaries green)

Testing

Docs-only change (6 markdown files, no Go touched).

• Secret scan on new files: clean.
• git status: clean, single commit in PR.
• go vet ./internal/... : blocked by environment (private GrayCodeAI/harrier + shrike modules return Repository not found) - pre-existing, unrelated to this change.
• markdownlint-cli2: not installed, skipped.
• Full make ci not run (module resolution blocked); needs CI run on PR.

Checklist

• My commits follow Conventional Commits (docs: ...)
• make build passes (not run - docs-only, CI will confirm)
• make lint passes (not run locally - CI will confirm)
• make test passes locally with -race enabled (not run - docs-only)
• New or changed code has tests (n/a - docs only)
• Public APIs have godoc comments (n/a - docs only)
• CHANGELOG.md updated under Unreleased if user-visible (n/a - release-please generates from commits; docs change)
• No secrets, tokens, or PII added to the repo
• No Co-authored-by: trailers