fix(install): stop folding sub-agent context into CLAUDE.md / copilot-instructions.md

[Minitour]

Summary

• Sub-agent context was being duplicated into the provider's primary instructions file (CLAUDE.md, .github/copilot-instructions.md) on top of the dedicated .claude/agents/{id}.md / .github/agents/{id}.md file. This bloated the main agent's context window on every turn.
• The fallback branch in writesSubAgentInstructionsContext (src/cli/utils/agents-file.ts:441) fired whenever the provider had a subagents integration AND an instructions filename that wasn't the universal AGENTS.md. That caught claude-code (the explicit foldSubAgentsIntoInstructions: false was silently overridden) and github-copilot. Providers writing to AGENTS.md (cursor, codex, …) were unaffected because the equality short-circuited the branch.
• After this change, a dedicated subagents integration is the single source of truth — no fold into the primary instructions file. Folding is only performed when a provider has no sub-agent integration AND explicitly opts in via foldSubAgentsIntoInstructions: true.

Behavioural diff

Provider	Before	After
claude-code	.claude/agents/{id}.md + CLAUDE.md snippet	.claude/agents/{id}.md only
github-copilot	.github/agents/{id}.md + .github/copilot-instructions.md snippet	.github/agents/{id}.md only
cursor / codex / gemini / … (AGENTS.md)	.{provider}/agents/{id}.md only	unchanged

capa clean continues to remove any stale sub-agent blocks from CLAUDE.md / AGENTS.md for users upgrading from a pre-fix install (the cleanup path is purely text-based and doesn't care how the blocks got there).

Test plan

• bun test src/cli/utils/__tests__/sub-agent-instructions.test.ts — flipped three assertions, added regression tests for claude-code and github-copilot
• bun test src/cli/utils/__tests__/agents-file.test.ts
• bun test src/cli/commands/__tests__/clean.test.ts
• Full suite: bun test (1096 pass, 0 fail)
• tsc --noEmit clean

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