fix: add dag-folder input and fix root-folder path matching

[daneeesh]

Summary

Fixes two bugs in the git diff change detection logic that prevented users from upgrading past v0.4:

• ./ prefix mismatch: root-folder: ./astro never matched git diff paths because git omits the ./ prefix. Normalizes root-folder (and the new dag-folder) by stripping ./ and trailing / before matching.
• DAGs outside root-folder invisible to change detection: Repos where DAGs live outside root-folder in version control (e.g., at dags/ in the repo root, copied into root-folder at CI time) had their DAG changes silently ignored. Adds a new dag-folder input to specify where DAGs live in git for change detection.
• Prefix collision in path matching: Switched from regex prefix matching (^"astro".*) to directory-exact matching ("$file" == "$ROOT_FOLDER"/*) so root-folder: astro no longer falsely matches astro-old/file.py. Applied consistently to root-folder, dag-folder, and DBT root-folder sections.

Fixes #103
Fixes #123

Usage for non-standard repo structures

# DAGs live at dags/ in repo root, Astro project at astro/
steps:
- name: Copy dags into astro folder
  run: cp -r dags astro/dags

- name: Deploy to Astro
  uses: astronomer/deploy-action@<version>
  with:
    root-folder: astro
    dag-folder: dags
    checkout: false
    deployment-id: <deployment-id>

Test plan

• dag-folder-deploy-test: validates that dag-folder: dags correctly detects dags/exampledag.py as a DAG change and triggers DAG-only deploy
• dag-folder-prefix-no-deploy-test: validates that dag-folder: dags does NOT match dags-old/exampledag.py (negative test for prefix collision)
• Existing e2e tests pass unchanged (backward compatible — dag-folder defaults to empty)
• Note: new e2e test jobs require Astronomer API tokens from secrets and need to be validated by a maintainer

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