Disable scheduled GitHub Actions workflows in forks

[ftnext]

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: Workflow fails when automatically executed on forks #3961

Problem:
Excessive notifications from periodic workflow runs (every 6 hours for triage, daily for stale-bot and docs upload) in forks where they are not needed.

Solution:
Add repository checks to prevent scheduled workflows from running in forked repositories.
The workflows will now only run in the main google/adk-python repository.

Testing Plan

I think that unit tests and E2E are not needed because this change is for GitHub Actions (not ADK source code).

Please describe the tests that you ran to verify your changes. This is required
for all PRs that are not small documentation or typo fixes.

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Please include a summary of passed pytest results.

Manual End-to-End (E2E) Tests:

Please provide instructions on how to manually test your changes, including any
necessary setup or configuration. Please provide logs or screenshots to help
reviewers better understand the fix.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• 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.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

[gemini-code-assist]

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