Problem

When an agent with auto_evolve = true creates or updates a skill via background_skill_review:

1. No approval gate for creates: the skill is installed directly without appearing in the Pending queue for human review (unless auto_evolve_mode = "controlled" is configured, which is not the default on existing agents)
2. No approval gate for updates/patches: when the reviewer proposes an update or version bump to an existing skill, it applies directly — no diff review, no approve/reject
3. Not added to creator's allowlist: agents with skills_mode = "allowlist" create skills they can never use — the new skill is globally installed but not in the agent's capabilities.skills list
4. Not visible in agent's Skills tab: the agent detail Skills tab only shows assigned skills, with no section for "available but not assigned" skills

Expected behavior

1. Auto_evolve creates → skill appears in Pending tab for approval
2. Auto_evolve updates/patches → skill version change appears in Pending with a diff view (old vs new) for approval
3. On approval, the skill is automatically added to the creating agent's allowlist (for new creates)
4. The agent's Skills tab shows a section "Available" with skills that exist globally but aren't assigned to this agent, with a + button to assign

Current behavior

• Creates silently install globally
• Updates/patches silently apply in-place
• Agent's allowlist unchanged after create
• Agent cannot use its own creation
• No approval, no visibility, no diff review for version changes

Suggested fix

Kernel

• approve_candidate in skill_workshop/storage.rs: after promoting to active, call registry.add_skill_to_agent(agent_id, skill_name) to update the creator agent's allowlist
• Store agent_id in the candidate provenance (already done)
• For updates/patches in controlled mode: create a candidate with the diff (old_content vs new_content) so the reviewer can see what changed before approving

Dashboard

• Pending tab: show diff view for update/patch candidates (old version vs proposed version)
• Agent Skills tab: add "Available" section below "Installed" showing unassigned global skills with assign/unassign buttons

Refs #5819, #5798