feat: add platform detection for Heroku, Platform.sh, and Upsun

[pjcdawkins]

Summary

• Add a platforms ruleset with rules detecting Heroku, Platform.sh, and Upsun cloud hosting platforms
• Heroku is detected via heroku.yml (definitive), app.json with buildpacks/addons (definitive), or Procfile alone (maybe)
• Platform.sh is detected via .platform.app.yaml/.yml or .platform/applications.yaml/.yml
• Upsun is detected via .upsun/*.yaml/.upsun/*.yml

Refs: AI-147

Test plan

• Added test fixtures for heroku-app, heroku-docker, and upsun-app directories
• Added assertions for all platform reports including existing configured-app (Platform.sh)
• make test passes
• make build regenerates expr.cache
• make gen_docs regenerates docs
• No new lint issues introduced (pre-existing issues only)

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR adds a new platforms ruleset to Whatsun’s YAML-driven analyzer so it can detect common cloud hosting platforms (Heroku, Platform.sh, Upsun) based on repository configuration files.

Changes:

• Introduces config/platforms.yml with detection rules for Heroku, Platform.sh, and Upsun (including a “maybe” Heroku rule for Procfile-only repos).
• Extends pkg/rules/analyze_testfs_test.go with new in-memory fixtures and assertions for platform detection reports.
• Regenerates generated artifacts (expr.cache, docs/functions.md) to reflect new expressions/docs output.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
config/platforms.yml	Adds the new platforms ruleset and detection heuristics for Heroku / Platform.sh / Upsun.
pkg/rules/analyze_testfs_test.go	Adds test FS fixtures and asserts expected platform reports from embedded rules.
expr.cache	Updates the prewarmed CEL expression cache to include new platform detection expressions.
docs/functions.md	Regenerated function/operator docs (generic type formatting updates).

[Copilot]

The Heroku app.json detection uses fs.fileContains to look for the substrings "buildpacks"/"addons". This will also match those words if they appear in values (e.g., description text) rather than as JSON keys, so it isn’t actually “definitive” and can produce false positives. Consider using jq(fs.read("app.json"), ...) to test for the presence of the keys (e.g., has("buildpacks") / has("addons"), or .buildpacks != null / .addons != null) instead of a raw substring search.

Suggested change

	&& (fs.fileContains("app.json", '"buildpacks"')
	|| fs.fileContains("app.json", '"addons"'))
	&& jq(fs.read("app.json"), 'has("buildpacks") or has("addons")')

[Copilot]

The heroku-procfile rule introduces new “maybe” behavior (Procfile-only projects should report heroku with maybe: true), but there isn’t a fixture/assertion covering this case. Add a Procfile-only test directory (no heroku.yml, no app.json with buildpacks/addons) and assert the platforms report includes Maybe: true (and is suppressed when a definitive cloud platform is detected in the same directory).