chore(followups): stub M13 + trim M8 DCGM section

[trilamsr]

Summary

Wave 2 PR-B per Lane H followups-dir recon. Two doc-only edits trim
~643 net LoC of stale carry-forward backlog from docs/followups/:

1. M13.md stubbed (446 → 13 lines, -433). Pyspy receiver work
   is DEFERRED to v0.4.0+ per #222
   (external-clock label) and RFC-0009. The receiver still ships in
   components/receivers/pyspy/, but the carry-forward queue is
   paused. File kept in place as a load-bearing marker — sibling
   issue [rc1-prep] Track pyspy-delete re-evaluation preconditions (blocked on #222 / RFC-0013 PR-M) #335 tracks re-evaluation preconditions.
2. M8.md DCGM section removed (318 → 108 lines, -210). RFC-0013
   §7 STRIKE'd the in-tree DCGM receiver in favor of dcgm-exporter
   • prometheusreceiver in the bundled recipe; the cgo stub never
     shipped real code. All DCGM-specific bullets (pkg/dcgm,
     client_cgo, kindWatch/kindMIG, AST resolver, libdcgm SIGSEGV
     subprocess, dcgm_info join-target, pkg/vendorsdk-template,
     perf-gate, HARDWARE-TESTING shipped marker) are gone. Surviving
     items: tracecore debug dump [AUDIT], validate --explain
     closed marker, prometheus-alerts federation labels [KEEP],
     grafana-dashboard per receiver [KEEP], OTel hw.* semconv
     upstream PR [UPSTREAM], build-tags CI closed marker, M8↔M9
     Option-pattern consistency review [AUDIT].

M3.md and M15.md strikethrough sweep was a no-op — rg '^~~'
returned zero matches in both files (the spec's "if 5+ matches,
collapse" predicate did not fire).

Pre-flight grep

rg DCGM docs/ cross-confirmed no inbound anchor refs into the
deleted M8 bullets — DCGM mentions in MILESTONES.md,
v1-rc1-cut-criteria.md, FOLLOWUPS.md, etc. are about the milestone
definitions and the RFC-0013 strike table, not specific M8 line
anchors. M11.md already documents that DCGM lives under M8 and is
[STRIKE] per RFC-0013 §7 — that pointer remains valid.

LoC delta

• Net delete: 643 lines (701 deletions − 58 insertions).
• M13.md: -433. M8.md: -210.

Conflict check

Different files than Wave 2 PR-A (docs/v1-rc1-*), Lane F
(module/**/*.go), and #504/#506. No overlap with any in-flight PR
touching docs/followups/.

Test plan

• git diff --stat confirms 2 files, 58 insertions, 701 deletions
• wc -l docs/followups/M13.md = 13 lines (target ≤25)
• grep -ic dcgm docs/followups/M8.md = 6 (down from 32)
• Pre-commit hooks pass (golangci-lint, go vet, attribute-namespace-check)
• DCO sign-off present

NONE