docs(runbooks): development, release, CI-triage and upstream-sync runbooks (2/5)

[sqr00t]

Closes #12.

Adds four operational runbooks under docs/runbooks/, plus an index. No code changes —
documentation only. Five files, 846 added lines, nothing deleted.

What is here

Runbook	Lines	Covers
README.md	17	Index: which runbook to reach for, and the unverified convention
local-development.md	185	Fresh clone to green tests, per-OS toolchain, the stale-extension trap, the tox matrix
release-and-publish.md	190	Pre-release checklist, tagging, the publish workflow, verifying the upload, rollback
ci-triage.md	230	A red check per workflow and job, with the fix for each
upstream-sync.md	224	Diffing against upstream bw, what may diverge, re-verifying goldens

What to check when reviewing

Follow one, do not read it. These are procedures, and the only real test is whether they work
when executed cold. local-development.md on a machine without the toolchain already installed is
the highest-value check.

Two specific things worth a second opinion:

• The stale-extension trap in local-development.md (§5). Editing C++ and re-running pytest
  without rebuilding silently tests the previously compiled _core — the change appears to do
  nothing. The runbook's stated fix should be the one you would actually use.
• The decision procedure in ci-triage.md for a failing regression test: is this a real model
  change or platform libm drift? That branch determines whether someone re-baselines a golden value
  or hunts a bug, so getting it wrong is expensive in both directions.

ci-triage.md §2 also documents the Linux-versus-macOS divergence class (GCC/libstdc++ against
libc++) that produced #7 and bf87e07, because it reads as CI flakiness the first time you hit it.

Stack

The stack is a single linear chain of six PRs; merge bottom-up. This PR is the fourth link in that
chain and the second of the five documentation PRs. It is based on docs/adr-records (#15), not on
main, so review it after the PRs below it land or the diff will read wider than it is.

main
 └── fix/gcc-cpp-compat            #10  compile fix + ColProxy constructor
      └── cpp/format-blocking      #21  .clang-format, pinned+scoped clang-format, job blocking
           └── docs/adr-records                    #15  nine ADRs
                └── docs/runbooks                  #16  four runbooks   ← this PR
                     └── docs/roadmap-and-rust-port #17  roadmap + Rust assessment
                          └── docs/entry-documents  #18  README/CONTRIBUTING/ONBOARDING
                               └── docs/cpp-format-policy #22  ADR 0010 + cpp-job corrections