fix(subagents): align investigator role wording

[bkyssn]

Problem

Built-in explorer, reviewer, and advisor inherit the active parent child-tool surface in ordinary mode, but their role descriptions and child system-prompt addenda described them as read-only. This made the model-visible role purpose conflict with the [inherited-tools] capability class and the effective tools reported after spawn. Fixes #514.

Value

Parents can select investigator roles from wording that matches the capability boundary actually enforced by Pi and OpenPI. Explicit role allowlists and Plan Mode remain the places that create a read-only child boundary.

Approach

• Remove read-only authority claims from the three built-in investigator role descriptions and system-prompt addenda.
• Align the Subagents role reference with the existing inherited-tool contract.
• Add a focused prompt contract that exercises capabilityClass() through the public roster, derives effective child tools with the existing helpers, and confirms spawn reporting excludes parent-only tools. It also keeps READ_ONLY_AGENT_TOOLS classified as read-only for explicitly bounded roles.

Validation

• node --test --experimental-strip-types tests/extensions/subagents/prompt.test.ts — 14 passed.
• node --test --experimental-strip-types tests/extensions/subagents/agent-types.test.ts — 22 passed.
• node --test --experimental-strip-types tests/extensions/shared/child-session.test.ts tests/extensions/workflows/execute.e2e.test.ts — 42 passed.
• bun run check — passed.
• bun run test — failed in two pre-existing, independently reproducible Plan Mode renderer cases: tests/extensions/plan-mode/result-rendering.test.ts leaves OSC 8 terminal-link escapes in rendered text. This diff does not touch Plan Mode or rendering code.
• Manual Pi TUI smoke — not run; the focused tests cover the model-facing role roster and spawn-result contract deterministically.

Impact

• User-visible behavior: investigator role wording and the Subagents reference now match ordinary-mode inherited tools.
• Model-visible context: the three child system-prompt addenda no longer claim a read-only capability they do not enforce.
• Runtime/lifecycle and persisted config/data: none.
• Compatibility: built-in implementer, custom roles that omit tools, explicit allowlists, parent-only exclusions, and Plan Mode restrictions retain their existing behavior.

[tt-a1i]

Reviewed at exact head cd359f2. Standards and Spec both pass with no actionable findings. The revised investigator wording now matches the runtime-enforced inherited-tool contract, while explicit allowlists and Plan Mode remain the real read-only boundaries. Focused contract tests and hosted CI are green.