fix(dashboard): use Link for navigation buttons to support Ctrl+Click

[GenerQAQ]

Why we need this PR?

Navigation action buttons (Messages, Tasks, Details, View Session, etc.) in dashboard list pages use router.push via onClick handlers. This prevents native browser link behaviors — users cannot Ctrl+Click / Cmd+Click / middle-click to open in a new tab, or right-click to copy link.

Describe your solution

Replace <Button onClick={() => router.push(url)}> with <Button asChild><Link href={url}> for all navigation-only buttons. This renders proper <a> tags with button styling, enabling native browser link interactions while keeping the same visual appearance.

Key decisions:

• Conditional rendering for disabled buttons: Buttons with disabled state (user page counts, task button) render as <Link> when enabled, plain disabled <Button> when not — since <a> tags don't support disabled.
• SkillList component: Added optional getSkillHref prop. When provided, skill cards render as <Link> instead of <div onClick>, and "View Files" becomes a Link button.
• Back buttons preserved as router.push: Buttons reading returnTo from query params at click time stay as router.push since URLs depend on runtime state.
• Non-navigation buttons unchanged: Delete, Config, Edit Meta, Create buttons remain as onClick handlers.

Implementation Tasks

• Convert session list buttons (Messages, Tasks) to Link — Dashboard + OSS
• Convert learning spaces list Details button to Link — Dashboard + OSS
• Convert user list buttons (Disks, Sessions, Agent Skills) to Link with disabled handling — Dashboard
• Convert learning space detail View Session button to Link — Dashboard + OSS
• Add getSkillHref prop to SkillList component — Dashboard
• Convert agent skills viewDetails button to Link — OSS
• Convert messages page Disk and Task buttons to Link — OSS
• Clean up unused useRouter imports

Impact Areas

• Dashboard
• Client SDK (Python)
• Client SDK (TypeScript)
• Core Service
• API Server
• CLI Tool
• Documentation
• Other: ...

Checklist

• Open your pull request against the dev branch.
• All tests pass in available continuous integration systems (e.g., GitHub Actions).
• Tests are added or modified as needed to cover code changes.