CI: fail on undefined variables, lint workflows, update actions

[jackspiering]

Description

Tightens the Compose validation and lint workflows.

• Compose validation:
  • "variable is not set" warnings are now errors.
  • The template is validated too.
  • A required variable that .env does not list is flagged.
  • The Tailscale Serve JSON must parse.
  • The paths: filters are removed, so the check runs on every pull request and can be made required.
  • The job is named Compose files, and the log shows the Compose version.
• Formbricks: the 41 variables that existed only as comments in .env are now empty values (this is what produced the warnings). The rendered Compose output is identical before and after.
• Linting: removes the paths-ignore blocks, so a PR that touches only LICENSE or .gitignore cannot stay "Pending". Adds a step that fails when the rumdl action version and its version: input disagree.
• persist-credentials: false on every checkout.
• New workflow-lint.yml (actionlint and zizmor, only on .github/** changes, with the same concurrency block as the other workflows), and dependabot.yml for the pinned actions.

Related Issues

• None.

Verification

• The new step, run locally with bash -eo pipefail against Compose v5.6.0 and v2.38.2 (the runner's version): exit 0 on this branch. Before the Formbricks fix it flags only Formbricks, with the 41 names. A broken Serve JSON and a renamed required variable are both flagged. EspoCRM needs at most 4 dummy variables.
• actionlint 1.7.12 and zizmor 1.30.1 report nothing on all workflows.
• Pins checked against upstream: actions/checkout v7.0.1, zizmor-action v0.6.4, and the actionlint sha256.
• All three workflows ran on this pull request and passed.

Checklist

• I have performed a self-review of my code and followed the templates structure.
• I have added verification that the stack works as expected.
• I have updated necessary documentation (e.g. frontpage README.md ).
• I have selected the correct label(s) for this PR.

Additional Context

From an external review of the repository (section 4).

• After merging, an admin can add Compose files to the required status checks of main. lint keeps its name, because branch protection requires it.
• Workflow lint should not be required.
• Not included: the repository conventions job and the weekly link check.