[#154] Support subagents as a component type

[bguidolim]

Summary

• Add first-class agent: shorthand support in techpack.yaml for Claude Code custom subagents (.md files in .claude/agents/)
• Follow the established CopyFileType pattern used by skill:, hook:, and command: — zero changes needed in executor, doctor, dry-run display, or artifact tracking (all generic)
• Full export pipeline support: discovery, interactive selection, manifest building, and YAML rendering

Test plan

• swift build compiles cleanly
• swift test — all 593 tests pass (5 new tests added)
• Create a test pack with agent: shorthand and run mcs sync --dry-run
• Run mcs sync and verify .md files land in .claude/agents/
• Run mcs doctor and verify agent file check passes
• Run mcs export and verify agents are discovered and round-trip correctly

Closes #154

[Copilot]

Pull request overview

Adds first-class support for Claude Code custom subagents (.claude/agents/*.md) as a tech pack component type, aligning with the existing CopyFileType-driven install/export pipeline.

Changes:

• Introduces agent as a ComponentType + CopyFileType, including global (~/.claude/agents/) and project (.claude/agents/) target directories.
• Extends external manifest parsing and shorthand YAML support to accept agent: and type: agent / fileType: agent.
• Updates export discovery, interactive selection, and manifest building/rendering to include agent files; adds/updates tests accordingly.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
Sources/mcs/Core/Environment.swift	Adds agentsDirectory to the environment path set.
Sources/mcs/TechPack/Component.swift	Adds .agent to ComponentType and CopyFileType, wiring directory resolution for install/doctor.
Sources/mcs/ExternalPack/ExternalPackManifest.swift	Adds agent component type + agent: shorthand resolution and ExternalCopyFileType.agent.
Sources/mcs/Export/ConfigurationDiscovery.swift	Discovers agent files from agents/ for both global and project scopes.
Sources/mcs/Export/ManifestBuilder.swift	Includes agent files in build specs, YAML rendering, and doctor hint comments.
Sources/mcs/Commands/ExportCommand.swift	Adds agents to discovery summary and interactive/non-interactive selection, passing through to BuildOptions.
Tests/MCSTests/ManifestBuilderTests.swift	Ensures manifest building includes agent components and updates expected component/file copy counts.
Tests/MCSTests/ExternalPackManifestTests.swift	Verifies YAML deserialization and agent: shorthand behavior for external manifests.
Tests/MCSTests/ExternalPackAdapterTests.swift	Verifies adapter converts external agent copyPackFile components to internal definitions.
Tests/MCSTests/DerivedDoctorCheckTests.swift	Verifies derived file existence paths cover .agent correctly.
Tests/MCSTests/ComponentExecutorTests.swift	Verifies project-scoped install places agent files under .claude/agents/.

Comments suppressed due to low confidence (1)

Sources/mcs/ExternalPack/ExternalPackManifest.swift:316

• The shorthand-key doc comment above this type still lists hook, command, skill, etc. but not agent. Since agent is now supported, update that comment so the manifest authoring guide stays accurate.

        case hook          // Map — CopyFileShorthand (fileType: .hook)
        case command       // Map — CopyFileShorthand (fileType: .command)
        case skill         // Map — CopyFileShorthand (fileType: .skill)
        case agent         // Map — CopyFileShorthand (fileType: .agent)

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