feat(plugin): surface agent and parentAgent in plugin hook input

[ArmirKS]

Issue for this PR

Fixes #15403

Type of change

• New feature

What does this PR do?

Plugin hooks (tool.execute.before, tool.execute.after, shell.env) have no visibility into which agent made a tool call or who delegated it. The session layer already tracks parent relationships (Session.create sets parentID in task.ts), but this never reaches plugins.

This adds optional agent and parentAgent fields to the three hook inputs. When agent A spawns agent B via the task tool, hooks receive { agent: "B", parentAgent: "A" }. Top-level agents have parentAgent undefined.

The key line is in task.ts — parentAgent: ctx.agent. Everything else is threading it through existing Plugin.trigger call sites.

Same pattern as #13662 which added sessionID and callID to shell.env.

How did you verify your code works?

• 5 schema-level tests validating PromptInput, ShellInput, and UserMessage accept and preserve parentAgent (and remain backwards compatible without it)
• Typecheck clean across all 18 packages via bun turbo typecheck

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR