CI: split ci-cd.yml into tiers so docs/AI-only PRs skip the full matrix

[thehesiod]

Problem

ci-cd.yml currently runs the full matrix (6 Python versions × 2
OS × several variants = ~13+ test jobs, plus build, zizmor, coverage,
check) on every PR, regardless of what changed. That includes:

• docs-only changes (docs/**, *.md, *.rst)
• .github/workflows/claude.yml + prompt edits (.github/claude-*-prompt.md,
  plugins/aiobotocore-bot/**) — the AI-automation code path
• .github/workflows/*.yml edits that don't affect the library
• CHANGES.rst, CONTRIBUTING.rst, .pre-commit-config.yaml,
  pyproject.toml [tool.rumdl], .serena/, etc.

Running the full matrix for "fix a typo in docs/override-patterns.md"
burns ~15 CI minutes and blocks the merge queue for the sake of a
bytecode path the change didn't touch.

Proposal

Split CI into tiers driven by on.pull_request.paths / paths-ignore,
with a single umbrella check (check job / alls-green) that
aggregates per-tier results so the merge-queue status check name stays
stable.

Tier A — docs-only changes

Triggered when only paths in this set change:

• docs/**
• **.md, **.rst (except CHANGES.rst when aiobotocore/__init__.py
  also changed — treat as code then)
• CONTRIBUTING.rst, README.rst

Runs:

• pre-commit run --all (lint/format via ruff, yamllint, rumdl)
• make docs (Sphinx build)

Skips the Python test matrix entirely.

Tier B — AI automation / workflow / tooling changes

Triggered when only these change:

• .github/workflows/claude.yml
• .github/workflows/botocore-sync.yml
• .github/claude-*-prompt.md
• plugins/aiobotocore-bot/**
• .claude-plugin/**
• .github/usage-summary.py
• .github/ISSUE_TEMPLATE/**, .github/pull_request_template.md
• .pre-commit-config.yaml, pyproject.toml (only [tool.rumdl],
  [tool.ruff], [tool.uv], [tool.pytest.ini_options] sections —
  may be hard to detect precisely; fall back to Tier C if any non-tool
  section changed)
• .github/dependabot.yml, .codecov.yml, .yamllint, .readthedocs.yml

Runs:

• pre-commit run --all
• zizmor
• A single Python job (one version, one OS, core path only) to
  confirm the workflow/tooling changes didn't break the basic test
  harness. Not the full matrix.

Tier C — library code / dependency changes

Triggered when aiobotocore/**, tests/**, pyproject.toml
(non-[tool.*]), uv.lock, or anything else changes — this is the
default path and runs today's full matrix.

Implementation sketch

• Each tier becomes its own workflow (or one workflow with multiple
  top-level jobs + paths: filters).
• alls-green / check job requires whichever tier actually fired.
  Use allowed-skips in re-actors/alls-green@… to accept skipped
  Tier-C matrix when Tier A fired, and so on.
• The merge-queue required-status-check remains a single check name
  so branch protection rules don't need to change.

Alternative (simpler, less pure): use paths-ignore: on the existing
workflow to skip Tier-C on docs/AI paths, and add a lightweight
dedicated docs.yml / ai.yml for the other tiers. Same effect, less
refactoring.

Motivation

PR #1556 ran the full CI matrix ~7 times during its back-and-forth —
each run ~15 minutes of CI time — for a PR that is 99% documentation
and automation config with no library code changes. Across all the AI
workflow changes landing lately, this has amounted to hours of pointless
matrix runs.

The zizmor rate-limit failure we just debugged was partly caused by
setup-uv fetching releases on every one of those full-matrix runs;
reducing matrix runs for docs/AI PRs also reduces API-budget pressure.

Non-goals

• Don't reduce coverage for actual library changes. Tier C stays the
  current full matrix.
• Don't change test semantics. Just change when each tier fires.

Related

• PR AI workflows: docs, plugin packaging, rumdl linter, security hardening #1556 is the immediate motivator (docs + plugin + hardening).
• Optimize Claude CI workflows for efficiency #1544 optimized Claude CI workflows for cost; this is the ci-cd.yml
  equivalent.