docs: fix demo GIF script paths and add split-screen demo

[msampathkumar]

Summary

Fixes the recorded demo GIF (it showed No such file or directory errors) and adds a new split-screen demo of an A2A server + the a2a CLI side by side.

What changed

Fix

• docs/demo/demo.sh: cd to the repo root so the --exec example-script paths resolve regardless of the directory the recording is launched from. This is the root cause of the No such file or directory failures in the old GIF. Keeping the paths relative also keeps the agent card's Wraps command: line clean (no absolute home-dir path).
• docs/demo/a2a-demo.gif: regenerated from the fixed demo — now shows the real completed round trip instead of the errors.

Add

• docs/demo/demo2.sh + docs/demo/demo2.gif: a new split-screen tmux demo — the A2A server (built from a plain script, no A2A code) on the left, the a2a CLI discovering and talking to it on the right.
• examples/01-exec-demo/README.md: embeds demo2.gif.

Refactor

• docs/demo/run.sh: generalized with DEMO/GIF env vars (defaults unchanged — the original flow is byte-for-byte identical) plus an optional, guarded TRIM_TAIL step used by the split-screen demo.

Verification

• No Go code touched; the Go workflow is path-filtered and does not apply.
• Both demos were run end-to-end from an arbitrary CWD: no errors, no home-dir leak in the rendered cards.

Regenerate

# demo 1 (single screen)
docs/demo/run.sh
# demo 2 (split screen)
DEMO=docs/demo/demo2.sh GIF=docs/demo/demo2.gif COLS=150 ROWS=28 TRIM_TAIL=1 docs/demo/run.sh

Rendered GIFs

Split-screen (demo2.gif):

Single-screen, fixed (a2a-demo.gif):

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Draft — opening for review of the demo content and the run.sh generalization.

[msampathkumar]

LGTM