test(coverage): add unit tests across all packages; remove dead code

- Added comprehensive table-driven test suites for all packages:
  cmd/glint, cicontext, fetcher, graph, linter, model, resolver.
  Coverage reaches 98%+ statement coverage across the codebase.
- Replaced os.Exit calls in cmd/glint with an `exit` variable so tests
  can capture exit codes without terminating the test process.
- Removed unreachable code found during coverage analysis:
  dead guard in cicontext.parseRegexLiteral; dead len(jobs)==0 branch
  in graph.Pipeline; skipWin struct field and dead continue in
  graph.convertToPNG; pipelineSVG return type simplified to string.

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

CHANGELOG.md

@@ -5,6 +5,17 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 This project uses [Semantic Versioning](https://semver.org).
 
+## [0.2.21] - 2026-06-14
+
+### Changed
+
+- **Internal:** replaced all `os.Exit` calls in `cmd/glint` with an `exit` variable to enable unit testing without process termination; no behaviour change.
+- **Internal:** removed unreachable code paths found during coverage analysis — dead guard in `cicontext.parseRegexLiteral`, unreachable `len(jobs) == 0` branch in `graph.Pipeline`, and the `skipWin` struct field / dead `continue` in `graph.convertToPNG`; `pipelineSVG` return type simplified from `(string, error)` to `string` as it never returned a non-nil error. No behaviour change.
+
+### Added
+
+- **Unit test suite** — comprehensive table-driven tests added across all packages (`cmd/glint`, `internal/cicontext`, `internal/fetcher`, `internal/graph`, `internal/linter`, `internal/model`, `internal/resolver`), reaching 98%+ statement coverage.
+
 ## [0.2.20] - 2026-06-14
 
 ### Added