chore(ci): remove scheduled full CI from ci.yml

[c-vigo]

Description

Removes the nightly schedule trigger from .github/workflows/ci.yml and all schedule-only checkout ref overrides, as agreed in #492. PR and workflow_dispatch behavior is unchanged. Updates security-scan.yml header comments so they no longer reference the removed 04:00 UTC CI run. Changelog for 0.3.2 is adjusted: drop the unreleased “Nightly CI schedule” (#461) bullet from Changed and document the removal under Removed (#492).

Type of Change

• feat -- New feature
• fix -- Bug fix
• docs -- Documentation only
• chore -- Maintenance task (deps, config, etc.)
• refactor -- Code restructuring (no behavior change)
• test -- Adding or updating tests
• ci -- CI/CD pipeline changes
• build -- Build system or dependency changes
• revert -- Reverts a previous commit
• style -- Code style (formatting, whitespace)

Modifiers

• Breaking change (!) -- This change breaks backward compatibility

Changes Made

• .github/workflows/ci.yml — Remove schedule (cron: '0 4 * * *'); remove nightly trigger from file header; drop with: ref: ${{ github.event_name == 'schedule' && 'dev' || github.ref }} from all checkout steps (default ref for PR/dispatch).
• .github/workflows/security-scan.yml — Header: describe PR-based full CI + Trivy; remove “after nightly CI at 04:00 UTC”; keep nightly 05:00 UTC scan behavior unchanged.
• CHANGELOG.md / assets/workspace/.devcontainer/CHANGELOG.md — Remove Nightly CI schedule ([CHORE] Add nightly CI and upgrade scheduled security scan #461) from Changed; add Nightly full CI schedule from ci.yml ([CHORE] Remove scheduled CI build+test from ci.yml #492) under Removed with sub-bullets.

Changelog Entry

Release section ## [0.3.2] - TBD (this branch targets release/0.3.2, not ## Unreleased):

Removed

• Nightly full CI schedule from ci.yml (#492)
  • Remove the schedule trigger and schedule-only checkout overrides; CI remains on pull requests and workflow_dispatch only
  • Nightly GHCR :latest scan in security-scan.yml is unchanged

Changed (edit relative to prior 0.3.2 draft)

• Removed the Nightly CI schedule (#461) bullet from Changed so the release notes no longer claim a nightly full CI run.

Testing

• Tests pass locally (just test)
• Manual testing performed (describe below)

Manual Testing Details

N/A — workflow YAML and changelog only; validation via CI on this PR.

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation accordingly (edit docs/templates/, then run just docs)
• I have updated CHANGELOG.md in the [0.3.2] - TBD section (and pasted the entry above)
• My changes generate no new warnings or errors
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published

Additional Notes

N/A

Refs: #492