…ove context handling

Add Symfony to the question models and handlers so that detected
Symfony projects get proper configuration instead of falling back
to GenericStack or redirecting to Symfony CLI.

In non-interactive mode (as used by source-integration-apps), Symfony
projects now produce a config with symfony-build/symfony-deploy hooks,
/var/cache and /var/log tmp mounts, and public/ web root.

In interactive mode, the Symfony CLI prompt is preserved. If declined,
the stack is now set to Symfony instead of Generic.

Changes:
- models/stack.go: add Symfony constant, Title, RuntimeForStack (PHP)
- models/answer.go: add Symfony case in getStack()
- stack.go: set Symfony stack in both non-interactive and interactive
  decline paths
- build_steps.go: use Symfony Cloud configurator (symfony-build)
- deploy_command.go: use symfony-deploy
- mounts.go: /var/cache and /var/log as tmp mounts
- locations.go: root: public, passthru: /index.php

Closes [SIA-20: Implement Symfony configuration in platformify](https://linear.app/platformsh/issue/SIA-20/implement-symfony-configuration-in-platformify)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Keep fork's struct-based Runtime/ServiceName (loaded from registry.json)
over main's string constants. Keep fork's simplified templates without
BuildFlavor conditional. Remove version.go and platformifier_test.go
(superseded by fork's registry.go/registry.json and discovery tests).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>