feat(subagent): add opt-in bounded discovery profile

Problem

A coding subagent can burn its context budget before editing by issuing several unbounded full-file Read calls in parallel. Prompt-only guidance is insufficient: the worker may interpret a list of relevant files as a request to read each entire file.

Proposal

Add an opt-in Subagent discovery profile that enforces context-efficient exploration at the tool boundary:

• require Read calls to specify offset and limit, with a deliberate escape hatch for explicitly approved small/full-file reads;
• cap lines and/or bytes returned per read;
• serialize reads during discovery so several large tool results cannot land in one turn;
• retain Glob, Grep, and targeted Edit;
• return an actionable rejection directing the child to Grep first and then make a bounded read;
• expose per-turn read/result byte and token accounting in the child transcript or diagnostics.

The profile should be opt-in, not a global default: full-file review and small files are legitimate workflows.

Motivation

A real delegated refactor consumed 274,477 input tokens and only 561 output tokens in one turn after five broad reads, then produced no implementation. The main failure was tool-result ingestion, not reasoning or output.

Acceptance

• A bounded-profile child cannot issue an unbounded Read or several concurrent reads.
• Rejections explain the bounded-read workflow.
• The profile does not change default Subagent behavior.
• Tests cover the escape hatch and read-volume telemetry.