feat(plugins): add Claude Code marketplace plugin compatibility

[Zetkolink]

Summary

Add full compatibility with Claude Code marketplace plugin directory structure, enabling Forge to discover, install, and run plugins designed for Claude Code's ecosystem.

Context

When installing a Claude Code marketplace plugin (e.g. claude-mem from ~/.claude/plugins/marketplaces/thedotmack), Forge showed all components as 0/none despite the plugin containing skills, hooks, MCP servers, and modes. The root cause: Claude Code marketplace repositories use a nested layout where marketplace.json points to the actual plugin subdirectory (e.g. ./plugin/), but Forge only looked for components at the repository root.

Additionally, Claude Code plugins reference $CLAUDE_PLUGIN_ROOT, $CLAUDE_PROJECT_DIR, and $CLAUDE_SESSION_ID in their hook commands and MCP server definitions — variables that Forge didn't provide.

Changes

Marketplace Discovery (Runtime + Install)

• Added MarketplaceManifest / MarketplacePluginEntry domain types for parsing marketplace.json
• Extended scan_root to detect marketplace indirection: when a directory lacks a direct plugin manifest but contains marketplace.json, the loader follows the source field to find the real plugin
• Added container directory scanning for cache/ and marketplaces/ layouts used by Claude Code's plugin manager
• Updated forge plugin install to resolve marketplace structure, count components from the effective plugin root, and copy only the plugin subdirectory (not the entire repo)

MCP Sidecar Counting

• Trust prompt now counts MCP servers from .mcp.json sidecar file in addition to plugin.json manifest — matching how the runtime loader already works

Environment Variable Aliases

• Injected CLAUDE_PLUGIN_ROOT, CLAUDE_PROJECT_DIR, and CLAUDE_SESSION_ID as aliases alongside their FORGE_* counterparts for both hook subprocesses and MCP server processes
• Claude Code plugins referencing ${CLAUDE_PLUGIN_ROOT} in hook commands now work under Forge without modification

Modes Display

• Added modes count to trust prompt, /plugin info, and /plugin list output — Claude Code plugins often ship dozens of operational modes

Flaky Test Fixes

• test_format_path_for_display_no_home: explicitly set home to None instead of relying on Faker-generated random value that could collide with test paths
• test_hook_exit_before_prompt_response_does_not_hang: increased elapsed threshold from 3s to 4s to account for parallel test load while still validating no-hang behavior

Use Cases

• forge plugin install ~/.claude/plugins/marketplaces/thedotmack correctly resolves the nested plugin, showing accurate component counts (skills: 7, hooks: present, MCP servers: 1, modes: 36)
• Runtime discovery of ~/.claude/plugins/ automatically finds plugins inside marketplaces/<author>/ via marketplace.json indirection
• Hooks using ${CLAUDE_PLUGIN_ROOT}/scripts/worker.cjs execute correctly with the variable resolved to the plugin's installed directory

Testing

# Run all new and affected tests
cargo nextest run --no-input-handler --lib -p forge_domain -- test_parse_marketplace
cargo nextest run --no-input-handler --lib -p forge_repo -- marketplace
cargo nextest run --no-input-handler --lib -p forge_main -- test_marketplace
cargo nextest run --no-input-handler --lib -p forge_main -- test_mcp_sidecar
cargo nextest run --no-input-handler --lib -p forge_services -- hook_runtime::env
cargo nextest run --no-input-handler --lib -p forge_services -- mcp::manager::tests

# Previously flaky tests — should now be stable
cargo nextest run --no-input-handler --lib -p forge_main -- test_format_path_for_display_no_home
cargo nextest run --no-input-handler --lib -p forge_services -- test_hook_exit_before_prompt_response_does_not_hang

23 new/updated tests added across 6 crates.