docs: agent discovery roadmap with testable P0/P1/P2 tasks

[kioie]

Summary

• Adds docs/AGENT-DISCOVERY-ROADMAP.md restructuring PostHog parity items into smaller, testable tasks
• Each task includes architecture notes (OSS vs cloud), prerequisites, file-level steps, required tests, DoD, PR scope, and rollback
• Adds wave-based execution order, dependency graph, and empty PR execution log table

Test plan

• Doc-only change — no code impact
• CI Build & test should pass unchanged
• Review open-core boundary notes per task

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[kioie]

Merge sync complete

Merged `main` into `docs/agent-discovery-roadmap` (includes #76 AGENTS-snippet). All CI checks on the latest push are green:

• Build & test ✅
• CI action smoke ✅
• Validate CHANGELOG ✅
• Security (Secret scan, Dependency review) ✅
• SonarCloud Code Analysis ✅

`gh pr merge 74 --squash` reports blocked by base branch policy despite green checks — likely queued third-party status contexts (DigitalOcean/Cursor/Claude) or ruleset gates not visible in `gh pr checks`. Retrying merge when those complete.