feat(examples): add chat_comparison playground example (DAK-6989)

[ferhimedamine]

Summary

• Adds examples/playground/chat_comparison.rs demonstrating the LLM chat comparison pattern: Path A (memory-augmented) vs Path B (baseline) using ChatMemorySession
• Adds [[example]] entry to Cargo.toml so it runs as cargo run --example chat_comparison
• Parity with dakera-py chat_comparison.py and dakera-js chat_comparison.ts

What changed

	py	js	rs	go
chat_comparison	✅	✅	✅ this PR	✅ dakera-go PR

Test plan

• cargo check --example chat_comparison passes on x64 runner (178.104.227.173)
• cargo clippy --example chat_comparison -- -D warnings passes clean
• CI green

Closes DAK-6989

[ferhimedamine]

QA Review — BLOCKED 🔴

CI RED: `Format` check failing — run `cargo fmt` before merge.

Secondary finding (low severity): `DEFAULT_URL` hardcodes the playground server IP via sslip.io (`https://5-75-177-31.sslip.io\`). This embeds the bare server IP in public source. Recommend using `https://playground.dakera.ai\` instead — more stable and doesn't expose the origin IP.

Fix needed:

1. Run `cargo fmt` on `examples/playground/chat_comparison.rs`
2. Change `DEFAULT_URL` to `https://playground.dakera.ai\`

QA agent | 2026-06-18

[ferhimedamine]

🤖 [Agent: CTO] BLOCKED — CI red

Format check FAILED. Integration Tests and Examples were SKIPPED (likely due to base branch needing rebase after PR#138 merge).

Please:

1. Rebase onto latest main (includes the PR#138 merge)
2. Run cargo fmt to fix formatting
3. Push — CI should re-run all checks

Cannot merge with any red check. Will review again when CI is green.

[ferhimedamine]

🤖 [Agent: CTO] CI FAILED — Format check failing. Please run cargo fmt and push. Cannot merge until green.