Add .golangci.yml with standard linters:
- errcheck, govet, ineffassign, staticcheck, unused, misspell
- gofmt and goimports formatters with local prefix

Fix lint issues found:
- Replace fmt.Sscanf with strconv.ParseInt in formatTimestamp
- Properly handle resp.Body.Close() errors using named returns
- Fix import ordering with goimports

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Add build-time version injection via ldflags:
- VERSION from git describe
- COMMIT from git rev-parse
- DATE from current time

Add new targets:
- test-cover: Run tests with coverage report
- test-short: Run quick tests
- lint: Run golangci-lint
- fmt: Run gofmt and goimports
- deps: Download and tidy dependencies
- verify: Verify go.mod

Update build target to output to bin/ directory.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Add version package with variables set via ldflags:
- Version: semantic version from git tag
- Commit: git commit hash
- Date: build timestamp

Includes Info() helper for formatted version string.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Add version information to root command:
- Set Version field from version package
- Custom version template showing commit and build date

Now `slack-cli --version` shows:
  slack-cli vX.Y.Z (commit: abc123, built: 2024-...)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Apply goimports formatting with local prefix to ensure consistent
import ordering across all command files.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Add NewWithConfig constructor for test injection with custom:
- Base URL (for httptest servers)
- Token
- HTTP client

Add 27 tests covering:
- Client configuration and initialization
- Channel operations (list, get, create, archive, unarchive, topic, purpose, invite)
- User operations (list, get)
- Message operations (send, update, delete, history, thread replies)
- Reactions (add, remove)
- Team info
- Pagination handling
- API error handling
- Network error handling
- Invalid JSON handling
- Auth header verification

All tests use httptest.NewServer for HTTP mocking.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Add 12 tests covering:
- GetAPIToken from environment variable
- GetAPIToken when no token configured
- IsSecureStorage platform detection
- Config directory resolution (default and XDG_CONFIG_HOME)
- Config file operations (set, get, delete)
- Multiple keys handling
- Key updates
- File permissions (0600 for file, 0700 for directory)
- Values containing equals signs

Uses t.TempDir() and t.Setenv() for isolated testing.
Skip keychain-dependent tests on macOS where appropriate.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Add tests for messages.go helper functions:
- formatTimestamp: timestamp parsing and formatting
- truncate: string truncation with newline handling
- buildDefaultBlocks: Block Kit structure generation

Add tests for command structure:
- Verify messages command registration
- Verify subcommands exist (send, update, delete, etc.)
- Verify command aliases (msg, m)

Uses table-driven tests with t.Run for clear test cases.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Add comprehensive manual test documentation covering:
- Test environment setup and prerequisites
- Test data conventions
- Command tests for all operations:
  - Configuration (set-token, show, delete-token)
  - Channels (list, get, create, archive, topic, invite)
  - Users (list, get)
  - Messages (send, update, delete, history, thread, react)
  - Workspace (info)
- Edge cases and error scenarios
- Pre-release test execution checklist
- Troubleshooting guide

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

This is a major restructuring to align with CLI guide best practices:

- Move main.go to cmd/slack-cli/main.go
- Create internal/cmd/root/root.go with root command
- Split each command file into internal/cmd/<resource>/ directory
  - channels: list, get, create, archive, unarchive, set_topic, set_purpose, invite
  - users: list, get
  - messages: send, update, delete, history, thread, react, unreact
  - workspace: info
  - config: set_token, delete_token, show
- Create internal/output package for shared output flag to avoid import cycles
- Update Makefile to build from new location
- Update .goreleaser.yaml with new main path and ldflags
- Update .gitignore to allow cmd/slack-cli/ directory

All tests pass and lint is clean.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>