Doctor: Port agent-neutral check to TypeScript

[normenmueller]

Context

Migrate the scaffold check cli/src/lib/doctor/_scaffold/checks/agent-neutral.sh to a native TypeScript module at cli/src/lib/doctor/checks/agent-neutral.ts.

Acceptance Criteria

• cli/src/lib/doctor/checks/agent-neutral.ts exports validate(corpusRoot: string): DiagnosticResult[]
• Check verifies that no capability artifact (.md or .meta.yaml) references a specific agent name
• Pattern matching replicates the shell script's grep logic in TypeScript
• ai4x doctor invokes the new check (replaces scaffold invocation)
• Scaffold counterpart (agent-neutral.sh) deleted after port
• Tests cover: clean capability (pass), capability mentioning agent name (fail), edge cases (agent name in distinguish_from context)

Technical Notes

• Pure function: corpus path in, diagnostics out
• DiagnosticResult schema as defined in the doctor module
• The shell script uses grep/find; TypeScript port must replicate the same glob + pattern logic
• Reference implementation: cli/src/lib/doctor/_scaffold/checks/agent-neutral.sh