feat(cli): multi-context simulation via --context flag

Add --context KEY=VALUE[,...] (repeatable) to glint check. When two or
more --context flags are given, glint evaluates every pipeline job across
all contexts and prints a side-by-side comparison table:

  glint check --context branch=main --context branch=develop .yml

Each column shows active / manual / skipped / blocked per job. Known
context keys are branch, tag, source (case-insensitive); any other
KEY=VALUE pair is treated as a CI variable override. The --changes /
--changes-from changed-file list is shared across all contexts. Implicit
branch=main / source=push defaults are skipped when --context is used.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -6,7 +6,7 @@
 
 <p align="center">
   <a href="LICENSE"><img src="https://img.shields.io/badge/license-Apache%202.0-blue.svg" alt="License"></a>
-  <a href="CHANGELOG.md"><img src="https://img.shields.io/badge/release-v0.2.21-blue.svg" alt="Release"></a>
+  <a href="CHANGELOG.md"><img src="https://img.shields.io/badge/release-v0.2.22-blue.svg" alt="Release"></a>
 </p>
 
 > **Disclaimer:** This tool was built through iterative AI-assisted development with [Claude](https://claude.ai). It is experimental, incomplete, and not intended for production use. Coverage of GitLab CI keywords is best-effort and may lag behind GitLab's evolving spec. Use it at your own discretion — no correctness guarantees are made. Contributions and bug reports are welcome.
@@ -17,7 +17,7 @@ A local tool to validate and lint `.gitlab-ci.yml` pipelines without needing a G
 
 - **Lints** — 43 rules covering pipeline structure, keyword constraints, `needs:`/`dependencies:` graphs, expression reachability, and deprecations (GL001–GL043); run `glint explain <ID>` for any rule
 - **Resolves includes** — local files, HTTPS URLs, GitLab project templates, and CI/CD Catalog components, with offline cache support
-- **Simulates context** — `--branch`, `--tag`, `--source` flags evaluate `rules:if:` and `only`/`except` to show which jobs would be active, manual, or skipped
+- **Simulates context** — `--branch`, `--tag`, `--source` flags evaluate `rules:if:` and `only`/`except` to show which jobs would be active, manual, or skipped; `--context branch=main --context branch=develop` prints a multi-column comparison table across multiple contexts in one run
 - **Multiple output formats** — `--format text` (default, ruff-style), `json`, `sarif` (GitHub Code Scanning / GitLab SAST), `junit`, `github` (PR annotations)
 - **Project config** — `.glint.yml` for rule suppression, severity overrides, token/URL defaults; `# glint: ignore RULE` for per-job inline suppression
 - **Graph visualization** — `glint graph` prints a terminal job tree; `glint graph pipeline` renders a GitLab CI-style SVG/PNG