refactor(cli): four install-use-cases and four capability classes duplicate the same pipeline

[blafourcade]

Problem

Two clusters of near-identical classes, confirmed by jscpd.

Cluster 1 — install use-cases

src/application/use-cases/install/install-agents-use-case.ts, install-rules-use-case.ts, install-skills-use-case.ts, install-commands-use-case.ts (325 lines combined) implement the same walk-filter-transform pipeline over contentFiles, differing only in which capability they read and one extra argument agents' convertFrontmatter takes.

Cluster 2 — capability classes

src/domain/capabilities/agents-capability.ts, commands-capability.ts, rules-capability.ts, skills-capability.ts duplicate the same 8-method surface (buildOutputPath, buildInstallPath, convertFrontmatter, reverseConvertFrontmatter, acceptsFileName, serialize, accepts, equals).

Why this isn't just cosmetic

The capability duplication has already drifted: AgentsCapability.acceptsFileName(fileName, allToolSuffixes) (agents-capability.ts:91) takes an externally supplied suffix list, while the other three compute ALL_TOOL_SUFFIXES internally at module scope for the same behavior — the same contract implemented two incompatible ways. The next person to touch one capability class has to know to check the other three.

Fix

1. Extract one generic InstallContentUseCase<C> parameterized by a capability accessor; keep the one real behavioral difference (agents passing relativeFileName) as an options flag.
2. Factor the capability classes' common surface into a shared base or composed helpers; keep only true per-capability differences (agents' toml/markdown format, skills' prefix mode) as overrides/options, fixing the acceptsFileName signature drift as part of the same change.

Acceptance criteria

• Install use-cases share one generic implementation, no behavior change
• Capability classes share a common base/helpers, acceptsFileName's signature is consistent across all four
• pnpm jscpd no longer flags these two clusters

Found by a code-quality audit of cli/ — full report: cli/aidd_docs/tasks/2026_07/2026_07_22_audit/code-quality.md.