Avoid full-suite fallback for ordinary documentation in affected PR tests #1177

The affected-test trial currently falls back to the full cross-platform suite for every non-Go file, including ordinary documentation. This defeats selective testing for PRs that include code/tests together with user-facing docs.

Concrete observation: PR #1172 at e6a8b4b799 changes CLI tests and recovery documentation. CI run 40879226996 selected full mode with reason full fallback for M: docs/cli.md, scheduling all indexer and store groups as well as CLI tests. There is no ci:full label or repository override on that run.

Extend selection narrowly for added/modified ordinary Markdown documentation under docs/. Validate the paths and supported package metadata so embedded or otherwise owned build/test inputs cannot be silently ignored. A mixed code/docs PR should retain the affected Go package closure; a verified docs-only PR may use the existing explicit empty selection. Keep full lint, builds and security checks.

Required controls:

• Mixed ordinary Go/test changes plus documentation select the same affected packages as the Go changes alone.
• Verified docs-only changes produce an explicit valid empty test selection.
• Embedded Markdown and uncertain ownership still fall back to full.
• Dependency changes (including Dependabot), CI/policy changes, unknown non-Go inputs and ci:full remain full; main always runs the full suite.
• Rename/delete/symlink or unsupported paths retain conservative behavior.
• Update the CI-testing documentation and run meaningful selector/runner/workflow tests and the prescribed pinned checks before pushing.

One issue, one isolated worktree and one PR. This is a focused improvement to the existing CI trial requested by the user; it does not waive release acceptance tests or change the currently running PR's plan.