chore: rename hawk-eco to graycode-eco, drop bundled skills

[Patel230]

Aligns the repo with the org name and the 'hawk owns no skills' model.

Rename (hawk-eco -> graycode-eco)

• Org/workspace is now graycode-eco; renamed all references across docs, templates, scripts, and code comments (excluding the literal hawk ecosystem CLI command, which is a real subcommand).
• Renamed port-plan + ecosystem-summary filenames and fixed all links.
• AGENTS.md: product is hawk (CLI + config dir ~/.hawk per code); graycode-eco is the workspace folder only.

Skills: hawk owns none by default

• Removed internal/plugin/bundled_skills/ (32 embedded skills) + the go:embed loader + tests + storage.BundledSkillsDir.
• Skills now come from the separate GrayCodeAI/hawk-community-skills repo via hawk skills search/install/list (on demand, user-initiated).
• Updated AGENTS.md, docs/user-guide/08-skills.md, cmd/diagnostics.go, cmd/root.go.

Dead-doc cleanup

• Removed unreferenced stale plans/audits (CODE_AUDIT_REPORT.md, PR_BODY.md, Y0-CALL-SITE-INVENTORY.md, monorepo-analysis.md, icon-survey.md, THEMING-ENHANCEMENT-PLAN.md, several architecture migration/verification docs).
• Removed dead vendored spec copies (spec/agent-skills, spec/spec-kit, spec/openspec/docs+examples+templates); kept the load-bearing spec/openspec/schema.yaml (embedded by internal/spec) and updated the indexes.

Verified twice: go build ./... + go vet + make lint (0 issues) + markdownlint (0 issues) + full make test (124 packages pass).