chore: add just launcher commands

[ovitrif]

This PR adds a Just command launcher for common local Android development tasks.

Description

This PR:

1. Adds a top-level Justfile with short commands for setup, build, test, lint, release, E2E, translations, changelog preview, and cleanup.
2. Adds an .env.example template and just init for creating the ignored local .env file.
3. Updates README, agent rules and release command docs to route thru the Just launcher.

Preview

N/A

QA Notes

Manual Tests

N/A

Automated Checks

• just init
• just list
• just --dry-run release
• just --dry-run test file LightningRepoTest
• just --dry-run test lane ComposeUi
• just --dry-run translations pull
• just --dry-run e2e network assembleTnetRelease
• just --dry-run e2e no geo
• just compile
• just test
• just lint
• just build
• just --unstable --fmt --check

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[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: f1c0320df4

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