docs: welcome contributors and scope agent guidance

[mehradotdev]

What changed and why?

Adds a focused CONTRIBUTING guide with minimal setup, optional-service boundaries, checks, and PR conventions; bug/feature issue forms; a short PR template; and README navigation.

Replaces .agents/rules/project-guideline.md with concise root/mobile/website AGENTS.md instructions. Detailed localization policy stays in docs/localization.md, and catalog guidance covers all existing catalogs. Existing release documentation stays in place. Also updates the Galaxy Store badge links in the README and website pages.

Closes #34.

Verification

• Rebased onto main after chore: add staged checks and pull request CI #87 was squash-merged; only the two remaining commits are included
• Both commits remain GPG-signed, and the rebase preserved the complete file contents
• Changed-file formatting and git diff --check passed
• All relative documentation links resolve; issue-form and workflow YAML parse with unique field IDs
• GitHub Actions checks passed on the rebased head
• Discussions is enabled with Q&A, Ideas, and Show and tell; existing bug, enhancement, documentation, good first issue, and help wanted labels are reused

Issue forms become available after merging to the default branch. Native/device verification is not applicable; no visual layout was changed.

• This PR is focused and its title follows Conventional Commits.
• Relevant documentation is updated; runtime tests are unaffected.
• Mobile user-facing text is not affected.

[coderabbitai]

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