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

[patoles]

Hi @donatoaz, sorry for the long silence on this, and thanks for the patience. I went through the PR and I'd like to move it forward: it builds on the v0.8.0 runtime abstraction correctly and still merges cleanly.

A few things before I can merge:

1. Could you sign the CLA? The bot link is above.
2. web/lib/agent-types.ts still has runtime?: 'claude' | 'codex', so Strands agents won't get a runtime badge/logo in the UI. Adding 'strands' to that union (and a fallback in the places that pick the brand mark) would complete the loop.
3. The session scan loop relies on the 10-minute ACTIVE_SESSION_AGE_S threshold to re-attach to stale sessions. Could you confirm the Strands watcher preserves that behavior (re-discovering a session whose file goes quiet and comes back)?
4. Heads up that v0.8.1 added anonymous telemetry; if it's easy, tagging Strands sessions with their runtime there would keep the stats meaningful.

On the strands-agentflow Python package: I'm open to it, but I'd like to think about where it should live long term (this repo vs its own). Happy to discuss in the PR.

Also interested in your pluggable-runtime idea from the comments; if this PR lands first, that refactor would have three concrete runtimes to generalize from, which seems like the right order.