[architect] refactor: delete dead skillreg BYO-agent contract (agentspec.go) and unused Resolve/List/Search lookups

[kubestellar-hive]

Refactor

Deletes pkg/skillreg's speculative, caller-less API surface, keeping only what production uses.

Deleted (zero callers outside own tests):

• src/pkg/skillreg/agentspec.go + agentspec_test.go — the whole BYO-agent SDK: AgentSpec interface, SpecData, AgentMode consts, ParseAgentSpec, LoadAgentSpec. docs/skills.md listed AgentSpec.DefaultSkills under 'Still not wired'; no launcher ever consumed it.
• Registry.Resolve and its private constraint grammar (satisfies, sameMajor, versionWildcard/caret/gte consts) — ResolveRequested resolves via Get (highest version wins), never Resolve.
• Registry.List, Registry.Search, skillMatches — nothing lists or searches skills.

Kept (the wired surface): NewRegistry, Add, Load, Get, ResolveRequested, InjectionText — exactly what the scheduler kick path (src/pkg/scheduler/scheduler.go:1593–1622, primeSkills) and dashboard skills status (src/pkg/dashboard/status_builder.go:485) call. versionParts/compareVersions stay (used by highestLocked).

Docs: docs/skills.md — package-surface table trimmed, constraint-grammar paragraph and 'Still not wired' section removed; docs/adr/0012-skill-registry.md — amendment noting the removal.

No behavior change. go build ./... clean; go test ./pkg/skillreg/... ./pkg/scheduler/... pass (dashboard's TestReposRescanRouteRequiresDashboardAuth failure is pre-existing/non-hermetic — see #6120).

Net: −487 lines.

Fixes #6200

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Filed by architect agent (ACMM L5 — hold-gated mode). Hold-gated: human review required.

— hive: agent=architect backend=copilot model=claude-opus-4-6

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