docs: add the Kotlin migration playbook and tracker

[nickolas-dimitrakas]

Summary

Adds the playbook and tracker for the Java to Kotlin migration of android-core and android-kit-base internals, and points AGENTS.md at them.

• docs/kotlin-migration/PLAYBOOK.md: the two governing rules (no breaking change; main is always releasable), what converts and what stays Java and why, the compatibility tiers and the check that proves each, the conversion rules that keep the compiled API identical, the per-PR checklist, risk classes and merge rules, the ProGuard protocol, rollback, and the steps to add when a release is cut while migration work is unreleased.
• docs/kotlin-migration/TRACKER.md: every planned pull request by phase and stack, with line counts, existing-test counts and risk class, plus the QA checkpoints.
• AGENTS.md: a short pointer to both documents.

Dependencies

The documents refer to tooling introduced by the other Phase 0 pull requests (apiCheck and scripts/check_api_dump.py, scripts/api_compat_report.py, settings-compat.gradle). This pull request should merge after them.

🤖 Generated with Claude Code

[github-actions]

📦 SDK Size Impact Report

What the SDK adds to a minified release APK.

Measured against an empty baseline app. Unlike the Rokt kit, android-core ships no Compose and no resources, so there is nothing here that a host app would already provide.

mParticle Core SDK

Metric	Target branch	This PR	Change
APK size	119.79 KB	119.79 KB	-2 bytes
Download size	117.61 KB	117.60 KB	-2 bytes
Dex bytes	213.75 KB	213.75 KB	0 bytes

➡️ SDK size impact change is minimal.

Raw measurements

Target branch:

{"baseline_dex_bytes": 0, "baseline_download_bytes": 2511, "baseline_install_bytes": 7527, "core_dex_bytes": 218884, "core_download_bytes": 122939, "core_install_bytes": 130192}

This PR:

{"baseline_dex_bytes": 0, "baseline_download_bytes": 2513, "baseline_install_bytes": 7529, "core_dex_bytes": 218884, "core_download_bytes": 122939, "core_install_bytes": 130192}

_{Measured 5fcb2ae merged into 6486216}

[cursor]

PR Summary

Low Risk
Documentation additions outlining the Java-to-Kotlin migration guidelines and roadmap with no changes to application code.

Overview
Adds documentation and tracking guidelines for the ongoing Java-to-Kotlin migration across android-core and android-kit-base.

Introduces docs/kotlin-migration/PLAYBOOK.md, which defines the rules to prevent breaking API changes, conversion best practices, review checklists, risk classifications, and ProGuard verification protocols.

Introduces docs/kotlin-migration/TRACKER.md to map out the planned migration phases, individual pull request stacks, line count baselines, and milestone checkpoints. Also updates AGENTS.md to direct contributors to these guidelines before modifying core or kit modules.

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