ci: modernize workflows

[2asoft]

Summary

Modernize the repository GitHub automation so CI can run on PRs and releases can publish to PlatformIO.

Changes

• Replace the stale push.yml workflow with .github/workflows/ci.yml.
• Build the ESP8266 and ESP32 examples with PlatformIO.
• Validate PlatformIO package metadata with pio package pack.
• Add .github/workflows/publish.yml for publishing GitHub releases to PlatformIO under the esphome owner.
• Add Dependabot configuration for GitHub Actions updates.
• Remove unused legacy CI helper scripts and old Probot stale config.

Verification

Ran locally:

pio package pack --output /tmp
python -m platformio ci -l . --board esp12e examples/FullyFeatured-ESP8266
python -m platformio ci -l . --board esp32dev examples/FullyFeatured-ESP32

All passed.