feat: add Strands Agents runtime support

[donatoaz]

Description

Adds support for Strands Agents as a runtime in Agent Flow, enabling visualization of Strands-based agent sessions alongside the existing Claude Code runtime.

• Introduces strands-agentflow Python package with a HookProvider that emits JSONL events compatible with Agent Flow's event protocol
• Adds a VS Code extension watcher (strands-session-watcher.ts) that monitors Strands session files and feeds events into the visualization
• Adds a runtime launcher (strands-runtime.ts) for starting Strands sessions from the extension
• Updates the relay script to support forwarding Strands events
• Includes unit tests and a sample fixture for the session watcher

Test plan

• Extension unit tests pass (32/32, including 4 StrandsSessionWatcher-specific tests)
• Project-level tests pass (24/24, no regressions)
• strands-agentflow Python package installs via uv and imports successfully
• JSONL events are correctly emitted to ~/.strands/agent-flow/<session-id>.jsonl
• Web app builds without errors (next build succeeds)
• VS Code extension builds without errors

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[donatoaz]

I also have a proposal to make supporting new runtimes more pluggable, this way the core stays unchanged and new runtime contributors adhere to an interace that keeps their contributions isolated and easier to test.. let me know if you are interested