Refactor docs: conventional README.md + LLM-optimized AGENTS.md

[Chuccle]

Summary

Replaces the sprawling root README.md (which mixed a project overview
with an ~1400-line performance investigation log) plus deploy/README.md
and deploy/DEBUGGING.md with a two-file structure:

• README.md — a conventional, concise, human-facing overview: what
  the project is, layout, build/test/deploy commands, and a pointer to
  AGENTS.md.
• AGENTS.md — the single home for all agent documentation and
  context (for any tool, not just one), reorganized for LLM consumption
  rather than straight-through reading:
  • A table of contents and a Quick reference section up front —
    commands, always-true rules, top footguns, and current read-ahead
    settings, each linking to its full section.
  • Sections ordered rules-first, reference-second: operating rules
    (what's expected, the full-pass checklist, documentation discipline,
    conventions — including a new rule on keeping the kernel call tree
    flat and being conservative with kernel stack usage) come early since
    they apply to every task; build/CI/sanitizers/deploy/debugging/perf
    reference material follows; the two large evidence/design sections are
    last since they're read occasionally, not every session.
  • deploy/DEBUGGING.md folded in as its own section rather than a
    separate file; overlapping content between it and deploy/README.md
    (e.g. the NMI bugcheck notes, the KdBreakPoint() writeup) consolidated
    into one place instead of two.
  • The ~1400-line playback-stutter investigation compressed into its
    conclusions, ruled-out hypotheses, the three built-and-reverted
    experiments, and the reusable measurement-methodology lessons — the
    full round-by-round measurement history remains recoverable from this
    branch's git history, not reproduced verbatim in the file everyone
    loads every session.
  • The on-disk hot cache design retitled "Future work: on-disk hot cache
    (not implemented)" with a banner making explicit it's an unstarted
    project with no code in the tree yet.
• CLAUDE.md removed — there is no per-tool file; Claude Code-specific
  notes (e.g. the optional Stop hook backstop) are folded into AGENTS.md.

Docs-only change; no source, build, or CI files touched.

Test plan

• Verified every cross-reference between the two files (and the
  section-reorder-broken "see X above/below" pointers within
  AGENTS.md) resolves correctly.
• Confirmed no other file in the repo (scripts, workflows) references
  the removed deploy/README.md, deploy/DEBUGGING.md, or
  CLAUDE.md, other than one comment in
  tools/Measure-BlorgScaling.ps1 which was repointed at AGENTS.md.
• Confirmed all Markdown table/code-fence formatting is well-formed.

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