Fix Go-version drift in AGENTS.md and enrich the README

[devantler]

🤖 Generated by the Daily AI Assistant

Part of the go-template roadmap #67.

Problem

Two documentation accuracy/completeness gaps:

1. Go‑version drift in AGENTS.md. The Project overview states the module "targets Go 1.24 (CI/release runners use the latest 1.25.x toolchain)", but go.mod now declares go 1.25.10. The floor was raised to 1.25.x (the reusable Dead Code Analysis installs deadcode@v0.43.0, which needs Go ≥ 1.25), so the "targets Go 1.24" line is stale and misleading to anyone reading the conventions.

2. Thin README. The README covers generic go mod/go build/go test commands but does not orient a newcomer to what the template actually gives them: the bundled tooling (golangci-lint v2 config, mega‑linter, pre‑commit, GoReleaser CD on v* tags, the org‑required validation workflows), the AI‑maintenance setup (AGENTS.md + the .claude/skills/maintain card), or how to adopt the repo as a template (ties into #CHILD_TEMPLATE).

Proposed direction

• Update AGENTS.md so the stated Go version matches go.mod (reference go.mod as the single source of truth rather than hard‑coding a number, so it can't drift again). While there, consider whether the go.mod floor should be the minor (1.25) rather than a specific patch (1.25.10) — pinning .10 forces every consumer's minimum to that exact patch; document the rationale either way (it's currently driven by tooling, not language needs).
• Enrich the README with a short "What's included" section (tooling + CI + release + AI maintenance) and the template‑adoption flow.

Acceptance criteria

• AGENTS.md no longer claims "Go 1.24"; the version statement agrees with go.mod (or defers to it).
• README has a concise What's included overview and points at the Use this template flow.
• mega-linter markdownlint/cspell stay green (add any new proper nouns to cspell.json).
• No behavioural/code change — docs only.

Size

XS–S — docs only.