docs(agents): rules-only rewrite of the os-dev agent definition (#15725)

* docs(agents): rules-only rewrite of the os-dev agent definition

Provenance narratives, ruling dates, verbatim quotations, incident
post-mortems and rationale tails leave the file; every rule keeps its
operative content, the section order, the frontmatter and the report
contract's pinned spellings. One rule per line, at most 120 bytes.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019RfFHiRCSs3JXLK4cwcfox

* tooling(pm): re-pin the os-dev.md line ceiling down at the landed count

469 to 403, one note in the map's own convention; no other row moves.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019RfFHiRCSs3JXLK4cwcfox

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Co-authored-by: Claude <noreply@anthropic.com>

Author: claude[bot]