docs: align CONTRIBUTING/README/roadmap with CalVer + dual-pace channels (ADR-0004)

[ChrisonSimtian]

Aligns the contributor- and consumer-facing top-level docs with the calendar-versioning + dual-pace channel model from ADR-0004. These are the living docs the earlier rollout PRs did not touch; they were still describing the old "any breaking change bumps the major / releases ship from release/vN / push to main publishes to nuget.org" world. PR 5 of the ADR-0004 rollout (after AGENTS.md, docs/agents/release-and-versioning.md, docs/branching-and-release.md, and the ADR itself).

Docs-only; no code changes. Wording/labels mirror docs/agents/release-and-versioning.md and docs/branching-and-release.md so nothing contradicts them.

Files changed

• CONTRIBUTING.md — main = edge channel, release/YYYY = stable train, release/v10 = legacy (security/critical only); target/YYYY labels (target/2026; legacy uses target/v10); breaking changes batched to the yearly major cut (land on main only, held for next year, breaking-change label + ⚠️ callout + next-major CHANGELOG entry), with the [Experimental("FALLOUT0xx")] escape hatch; two-tier review (light on edge, rigorous on promotion); Releases section now reflects edge→GitHub Packages on merge + stable from release/YYYY tags (nuget.org opt-in); release/vN→release/* in the CI-gate line. Also replaced the stale "don't fight .editorconfig/ReSharper settings" note (those files were removed during the takeover per AGENTS.md rule 6).
• README.md — reframed the v11/v12 IMPORTANT callout around CalVer + channels; added a "Channels" note under Install (stable CalVer from release/YYYY, edge prerelease from main to GitHub Packages, legacy 10.x semver); fixed the Build Status prose and the release workflow row trigger (was "push to main — publishes Fallout.* to nuget.org"). Global tool install (dotnet tool install -g Fallout.Cli, command fallout) left unchanged as instructed.
• docs/roadmap.md — added a CalVer versioning note up top; reframed "the next two majors" as theme-based milestones (rebrand + plugin foundation on the 2026 line; public plugin SDK in a later calendar major); dropped hard-coded v11/v12/v13 and 11.1 version numbers while preserving all the engineering/milestone content; tied breaking-changes-in-SDK-majors to the yearly cut and added the [Experimental] mention.

Version shape

README/CONTRIBUTING use the neutral 2026.MINOR.PATCH-edge.… form to avoid baking in a specific prerelease tail. I did not rewrite the date-stamped 2026.2.0-edge.<YYYYMMDD>.<h> shape used in release-and-versioning.md / branching-and-release.md — see the "left for human review" note below.

Deliberately left untouched (flagging for human review)

• Edge prerelease tail mismatch. The task brief says the real NB.GV output is 2026.1.0-edge.<height>.g<commit>, but the already-merged source-of-truth docs (release-and-versioning.md, branching-and-release.md) and ADR-0004 itself consistently and deliberately use the date-stamped 2026.2.0-edge.<YYYYMMDD>.<h> form (the ADR explicitly argues for date-in-prerelease-segment). I did not flip those docs to the gNNN shape — doing so would contradict the source-of-truth wording I was told to mirror, and the discrepancy looks like a deliberate decision rather than a typo. A human should confirm which prerelease tail is canonical and, if it's the gNNN form, fix all three docs (+ the ADR's runtime-shape references) in one consistent pass.
• docs/migration/from-globaltool-to-cli.md — references real shipped/unlisted package versions (Fallout.GlobalTool 10.3.40, Fallout.Cli 11.0.0, the unlisted 10.3.41-47 range, "v11 semver cleanup"). These are accurate history of a real migration event, not a versioning-model claim, so I left them. Renumbering them to CalVer would misstate what actually shipped.
• docs/agents/conventions.md line ~12 ("per-file headers stripped in v11") — dated historical fact, not a forward-looking versioning statement; left as-is.
• docs/architecture.md CI table — its release.yml | push to main | Publish to GitHub Packages row is actually edge-accurate and doesn't assert a versioning model, so I left it. A maintainer may still want to add the tag-triggered stable path for completeness.
• README NuGet version/downloads badges for Fallout.Common — live shields, not stale text; under the nuget.org-opt-in policy they may render "not found" until a stable release is pushed there. Left untouched per "don't overclaim/invent."
• The stale untracked vendor/ directory was ignored as instructed (not added or removed).

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