Add ToolActivityDescriber for real-time tool status

[myzie]

Summary

• Adds ToolActivityDescriber optional interface (raw json.RawMessage input) — backward-compatible, no required method on Tool
• Adds TypedActivityDescriber[T] for typed tools to implement with strongly-typed input
• TypedToolAdapter delegates to the underlying tool's ActivityDescription when present
• Agent's tool execution path checks for ToolActivityDescriber before PreToolUse hooks and populates HookContext.ActivityDescription
• Consumers attach a PreToolUseHook and read hctx.ActivityDescription to display status

Toolkit implementations (spec table):

Tool	Example output
BashTool	"Running: go test ./..."
ReadFileTool	"Reading src/agent.go"
WriteFileTool	"Writing src/agent.go"
FetchTool	"Fetching https://example.com"
GlobTool	"Searching **/*.go"
GrepTool	"Grepping for 'CreateResponse'"
ListDirectoryTool	"Listing toolkit/"

Implements tech spec: docs/tech-specs/tool-activity-description.md

Test plan

• go test -run TestToolActivityDescriber . — adapter delegation and fallback for non-implementing tools
• go test ./... — full suite passes

🤖 Generated with Claude Code

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch worktree-feat+tool-activity-description

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