Move CI to the shared Namespace runner

[leynos]

Summary

This branch moves repository-owned Linux jobs to the shared uncached
Namespace profile namespace-profile-default (Ubuntu 22.04, amd64, 4 vCPU,
16 GB) and records the custom runner labels for actionlint. Profile cache
volumes remain disabled. Native wheel matrices and externally controlled
reusable workflows retain caller-selected runners.

Review walkthrough

• Start with .github/workflows/ci.yml to review the runner contract.
• Start with .github/actionlint.yaml to review the runner contract.
• Start with docs/developers-guide.md to review the runner contract.

Validation

• actionlint: passed
• make check-fmt markdownlint: blocked because repository dependencies are not installed (node_modules/@biomejs/biome/bin/biome missing)

Notes

The shared profile is deliberately used without a repository-specific cache
volume during the pilot. The native wheel matrix remains GitHub-hosted because
it selects Windows, macOS, and architecture-specific runners at the reusable
workflow boundary.

Summary by Sourcery

Move repository-owned CI onto the shared uncached Namespace runner while documenting its contract and stabilizing related checks.

Bug Fixes:

• Make startup-output snapshots resilient to Node.js warning lines and process-specific output.

Enhancements:

• Move the repository-owned CI verification job to the shared uncached Namespace runner profile.
• Document the shared CI runner contract and configure actionlint with the Namespace runner labels.
• Refine spelling and documentation style checks for repository terminology and hexadecimal identifiers.

CI:

• Add actionlint configuration for the shared Namespace runner labels.

Documentation:

• Document the shared Namespace CI profile, its resource characteristics, disabled cache volume, and reusable-workflow runner behavior.

Tests:

• Update startup-output snapshots and normalization to accommodate current Node.js output.

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Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

The PR migrates the repository-owned Linux verification job to the uncached shared Namespace profile, adds actionlint awareness of the custom runner labels, and documents the runner contract while leaving native wheel matrices and externally controlled reusable workflows on caller-selected runners.

File-Level Changes

Change	Details	Files
Move the repository-owned verification job to the shared uncached Namespace runner profile.	Replace the GitHub-hosted Ubuntu runner with namespace-profile-default. Document the profile’s Ubuntu 22.04 amd64 environment, capacity, and disabled cache volume.	.github/workflows/ci.yml docs/developers-guide.md
Declare shared Namespace runner labels for actionlint validation.	Add the default amd64 and arm64 Namespace profile labels to the actionlint configuration.	.github/actionlint.yaml

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[leynos]

Where this branch's work went, so nothing is lost if it is eventually closed.

The Namespace migration is cancelled, so the runner change in c1a75be is not
being carried anywhere. The other commit is:

Commit	Where it is now
e502ca6 "Stabilize documentation and startup gates"	cherry-picked into #56 with your authorship preserved, and merged to main as part of f6565d3.

#56 also corrected three things in that commit, in a separate commit so the
difference stays visible in the history: ba had been added to [words] accepted, which stopped ba being corrected to be or by anywhere in the
repository, so the abbreviated commit hashes it answered are now matched by
their written form instead; the `color` example was restored to the style
guide and the term recorded in typos.local.toml; and the list marker on that
line, which had become +, failed markdownlint with MD032, which this
repository's CI does not run.

This pull request is deliberately left open. Closing it is yours to decide, not
an agent's.