Add a doctor advisory that suggests ISO 29148 decomposition for oversized PRDs

[ivklgn]

Problem

A large PRD is a signal that a project might be better served by the ISO 29148 cascade, but nothing surfaces that.

Expected result

A non-fatal advisory that flags an oversized PRD and names the ISO track. Conservative enough that it stays quiet on ordinary documents.

Where it belongs — decide this first

v0.7.0 added a second candidate home. There are now two:

• cmd/doctor.go — runs on demand, so a false positive costs one line in a report the user asked for.
• internal/advisory/ — the post-write advisories (precision, staleness) that hooks emit into the agent's context after a document mutation. It fires at the moment the PRD is written, but a false positive there spends tokens in every session.

internal/advisory/precision.go already parses a document body and reports findings, so the mechanics exist on that side. Pick one before implementing.

Scope

• Decide the host (doctor vs. post-write advisory).
• Define a conservative size heuristic and document it.
• Emit an advisory, never an error.
• Skip when requirements_approach is already iso (see Add a requirements_approach setting to settings.json #11).
• Test the trigger and the skip case.

Files: cmd/doctor.go or internal/advisory/ (+ tests)

Acceptance: the advisory appears for an oversized PRD and stays silent otherwise.

Stop/UserPromptSubmit hooks were removed once because of false positives. Keep the threshold conservative and the wording advisory.