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>

FEATURES.md

@@ -151,6 +151,24 @@ Use `--list-vars` to print the resolved variable table to stderr.
 | `--changes PATH` | Mark PATH as changed; repeatable |
 | `--changes-from REF` | Run `git diff --name-only REF` to auto-detect changed files |
 
+**Multi-context comparison** (`--context`):
+
+Pass `--context KEY=VALUE[,...]` (repeatable) instead of `--branch`/`--tag`/`--source` to evaluate every job across multiple contexts simultaneously. Each `--context` flag defines one column; glint prints a table showing `active`, `manual`, `skipped`, or `blocked` per job per context.
+
+Known context keys: `branch`, `tag`, `source` (case-insensitive). Any other `KEY=VALUE` pair is injected as a CI variable override. The `--changes`/`--changes-from` file list is shared across all contexts.
+
+```
+glint check --context branch=main --context branch=develop .gitlab-ci.yml
+
+Context comparison:
+
+JOB         branch=main  branch=develop
+----------  -----------  -------------
+build-job   active       active
+deploy-job  active       skipped
+test-job    active       active
+```
+
 ---
 
 ## Output formats