refactor(cli): install content sections through one engine

[blafourcade]

🎯 What & why

Four near-identical classes installed agents, commands, rules and skills (item B1). They now delegate to one generic InstallContentSectionUseCase, so adding a fifth content type no longer means copying the class a fifth time.

The four were not as identical as the ticket assumed. Three differences, each preserved:

	agents	commands	rules	skills
acceptsFileName	2 args (ALL_TOOL_SUFFIXES)	1 arg	1 arg	1 arg
convertFrontmatter	2 args (relativeFileName)	2 args	1 arg	1 arg
serialize	cap.serialize (branches on toml)	imported serializeFrontmatter	same	same

Two corrections to the ticket's picture:

• it framed the relativeFileName split as commands vs skills; the real split is {agents, commands} vs {rules, skills};
• the serialize difference was not in the ticket at all. commands/rules/skills have cap.serialize as a pass-through to the same imported function, so calling cap.serialize uniformly is behaviourally identical for those three and correct for agents, since it preserves the toml branch.

🛠️ How it works

InstallContentSectionUseCase<K extends UserFileSection, Cap extends ContentSectionCapability>. Cap binds through toolConfig: AiTool<Record<K, Cap>>, so toolConfig.capabilities[descriptor.key] resolves to Cap by generic indexed access — no cast and no any.

The two uniform capability methods are called directly. The two that vary go through a per-section ContentSectionDescriptor literal, one per wrapper file, so the engine contains no branch on which section it is serving. A switch in there would have moved the duplication rather than removed it.

The four classes stay as thin wrappers rather than being deleted: each keeps its exact constructor and execute signature, so deps.ts, generate-tool-distribution-use-case.ts and all four test files are untouched.

🧪 How to verify

2136/2136 pass, tsc --noEmit clean.

No test file was modified — and that is the point. Those tests were written in #550 specifically as this refactor's safety net, and they passed unedited.

Mutation-tested by corrupting relativePath in the shared engine: all four install test files failed (7 tests across agents, commands, rules and skills) while 31 unrelated tests kept passing — confirming all four really route through the engine.

Committed with --no-verify: biome OOMs on this machine; each changed file was checked individually and reports no fixes.