feat(pipeline): add when: conditional task execution

[albertodebortoli]

Description

Adds an optional when: field to pipeline tasks. When present, the runner evaluates the condition before executing the task — if the condition is false the task is skipped (a notice is printed, no error is thrown). This enables parameter-driven pipelines where certain tasks are optional, e.g. upload only when --param upload=true is passed.

Three expression forms are supported:

tasks:
  - name: Upload artifact
    command: upload --file build.zip
    when: ${upload} == true          # equality

  - name: Clean debug artifacts
    command: rm -rf debug/
    when: ${flavor} != release       # inequality

  - name: Run integration tests
    command: swift test --filter Integration
    when: ${run_integration}         # presence (truthy check)

${name} tokens are resolved from parameters (highest priority), then task env, then pipeline env. Unknown tokens resolve to "" (falsy). Falsy values: "false", "0", "no", empty string.

Dry-run output shows the condition and its evaluated outcome:

  Task 2: Upload
    Command: upload --file build.zip
    Tools:   upload ✓
    When:    ${upload} == true → skip

The pipeline summary counts only executed tasks (skipped tasks excluded).

Builds on the pipeline parameters feature (PR #82 — --param KEY=VALUE support).

Type of Change

• Feature

How Has This Been Tested?

• Added / updated unit tests
• Manually tested locally (describe)
• Tested on macOS (arch: arm64)

New test file:

• Tests/Core/TaskConditionEvaluatorTests.swift — 20 tests covering equality, inequality, presence, substitution, unknown tokens, whitespace trimming, malformed expressions

Extended existing tests:

• PipelineRunnerTests — 4 new tests: passing condition executes, failing condition skips, mixed skip/execute, nil when always executes
• PipelineLoaderTests — when: field decoded from YAML fixture

Checklist

• Swift code builds locally (swift build)
• Tests pass locally (swift test)
• Code style / formatting respected
• Documentation updated (README / comments)

Breaking Changes?

• No

Additional Notes

TaskConditionEvaluator is a new standalone component following the protocol-oriented DI pattern. PipelineRunner receives it via an internal init for test injection; the public init(printer:) uses the default implementation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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