feat: migrate qqbot to Plugin API v2

[kachofugetsu09]

What changed

• Declare Plugin API v2 explicitly.
• Split legacy initialization work across prepare(), activate(), retire(), and terminate() according to side-effect timing.
• Add the shared static contract workflow pinned to an immutable checker commit.

Compatibility

Depends on Core PR kachofugetsu09/akashic-agent#230. API v1 is intentionally not retained.

Validation

• Repository tests passed in an isolated worktree.
• Shared Plugin API v2 static contract passed.
• Core locked gate checked this exact commit from GitHub.
• Docker Debug atomic-reload, all-plugins, and fitbit phases all passed for the locked release set.