docs: Documentation analysis and improvement

> [!CAUTION]
> Protected files were modified in this change.
> This pull request is in `request_review` mode and requires explicit human scrutiny before merge.
>
> Protected files: `README.md`

Closes \#150

## Summary of files analyzed
- `README.md`
- `docs/RELEASE.md`
- `.default-oblt-cli-version`
- `hooks/pre-command`
- `hooks/pre-command.ps1`
- `plugin.yml`

## Issues found
- `README.md`: documented default `version` (`8.0.4`) drifted from `.default-oblt-cli-version` (`8.0.21`).
- `README.md`: credentials command was documented as generally supported while implementation exists only in `hooks/pre-command` (Bash).
- `README.md`: global git identity side effects (`git config --global user.name` and `user.email`) were undocumented.
- `docs/RELEASE.md`: release guide linked to non-existent workflow path (`bump-readme=version.yml`).
- `README.md`: wording used "both ... or" where behavior is "either ... or".

## Changes made
- Updated `README.md` `version` property default to reference `.default-oblt-cli-version` with current value (`8.0.21`) so docs remain aligned with the authoritative file and reduce future drift.
- Added a Bash-only support note for `command: credentials` in `README.md` to match current implementation and avoid misleading PowerShell users.
- Added a `Git identity behavior` section in `README.md` documenting global git config mutations and the `GIT_USER`/`GIT_EMAIL` overrides to make side effects explicit.
- Fixed the broken workflow link in `docs/RELEASE.md` from `bump-readme=version.yml` to `bump-readme-version.yml`.
- Corrected the README note wording from "both ... or" to "either ... or" for behavioral accuracy.

---
[What is this?](https://ela.st/github-ai-tools) | [From workflow: Observability Agentic Workflow — Schedule](https://github.com/elastic/oblt-cli-buildkite-plugin/actions/runs/34444278900)

Give us feedback! React with 🚀 if perfect, 👍 if helpful, 👎 if not.

---

> [!NOTE]
> This was originally intended as a pull request, but the git push operation failed.
>
> **Original error:** ERR_VALIDATION: pushSignedCommits: refusing unsigned push for branch 'docs/issue-150-documentation-analysis-improvement-9417b264ddb87dbf': Signed-commit payload violates file-protection policy (request_review): README.md
>
> **Workflow Run:** [View run details and download bundle artifact](https://github.com/elastic/oblt-cli-buildkite-plugin/actions/runs/34444278900)
>
> The bundle file is available in the `agent` artifact in the workflow run linked above.

To create a pull request with the changes:

```sh
# Download the artifact from the workflow run
gh run download 34444278900 -n agent -D /tmp/agent-34444278900

# Fetch the bundle into a temporary ref, then update the local branch
git fetch /tmp/agent-34444278900/aw-elastic-oblt-cli-buildkite-plugin-docs-issue-150-documentation-analysis-improvement.bundle refs/heads/docs/issue-150-documentation-analysis-improvement:refs/bundles/create-pr-docs-issue-150-documentation-analysis-improvement-9417b264ddb87dbf-e7b1b8a6
git update-ref refs/heads/docs/issue-150-documentation-analysis-improvement-9417b264ddb87dbf refs/bundles/create-pr-docs-issue-150-documentation-analysis-improvement-9417b264ddb87dbf-e7b1b8a6
git checkout docs/issue-150-documentation-analysis-improvement-9417b264ddb87dbf
# Ensure the working tree matches the updated branch
git reset --hard
# Remove the temporary bundle ref
git update-ref -d refs/bundles/create-pr-docs-issue-150-documentation-analysis-improvement-9417b264ddb87dbf-e7b1b8a6

# Push the branch to origin
git push origin docs/issue-150-documentation-analysis-improvement-9417b264ddb87dbf

# Create the pull request
gh pr create --title 'docs: Documentation analysis and improvement' --base main --head docs/issue-150-documentation-analysis-improvement-9417b264ddb87dbf --repo elastic/oblt-cli-buildkite-plugin
```

Review README.md and docs/RELEASE.md against .default-oblt-cli-version, hooks/pre-command, hooks/pre-command.ps1, plugin.yml, and the workflow references. Verify the documented version, credentials support, Git identity side effects, wording, and release link match the implementation, then confirm the documentation changes are ready for review.

Written by the indexing model from the issue text.