Manual: answer “what is a skill” directly and pin the four capability queries #1937

Follow-up to #1378 after checking dev at 3ba1d8f76. The original capability denial is addressed by the skills/manual work in #1410, but its exact retrieval acceptance is not fully covered.

Observed behavior

The current binary's manual command returns skills content for all four original questions. However, codeaf manual "what is a skill" begins with Which folders are read, followed by attachment and picture-exclusion sections, rather than a direct definition. The four verbatim questions are also absent from the retrieval regression table in internal/manual/chat_test.go.

The manual now correctly explains that skills work with automatic memory saving off; the old expectation that memory off disables skills must not be reintroduced.

Remaining work

1. Make "what is a skill" retrieve a direct definition and how a person can use one.
2. Add explicit retrieval probes for can you use skills, do you have skills, what is a skill, and why can't you use my skills.
3. Assert that each answer reaches the appropriate capability/definition/troubleshooting section, including the current memory-independent behavior.
4. Prove the probes catch a removed or unreachable answer section, and verify the same questions through the built binary's manual command.

This issue preserves the remaining retrieval and regression work from #1378; it does not report skills as unavailable.