docs: teach only the KEEP surface in agent skills

[pewpewpotato]

Refs #115

Agent skills and operator how-tos in this repo now teach the post-cut KEEP surface only. Marketplace (autonomous-bits/spool-marketplace) is out of scope for this PR, so this does not close #115.

Inventory

Rewritten

Skills (current how-to no longer names Rack, .spl as source of truth, or deleted CLI/MCP verbs):

• .agents/skills/spool/SKILL.md
• .agents/skills/spool/references/working-changes.md (export was “one-shot or repeatable”; it is now lossy, once, then stop)
• .agents/skills/spool/references/workspaces.md
• .agents/skills/spool/references/cli-help.md
• .agents/skills/spool/references/branches-and-history.md
• .agents/skills/spool/references/maintenance.md
• .agents/skills/spool/references/merges.md
• .agents/skills/spool/references/schemas.md
• .agents/skills/spool/references/batch-authoring.md
• .agents/skills/spool-technical-implementation/SKILL.md

Operator guidance that still listed deleted verbs as something to know:

• README.md
• docs/context-bind.md
• docs/architecture.md (deleted names point at internal/surface instead of a verb catalog)
• docs/README.md
• CONTRIBUTING.md
• cmd/spl/AGENTS.md
• internal/AGENTS.md
• CHANGELOG.md (Unreleased note that skills teach KEEP only)

Deleted

None. No skill existed whose only purpose was Rack or .spl-as-SoT. Bind, query, mutate, merge, and prune skills still map to KEEP work, so they were rewritten in place.

Reviewed, already KEEP (no edit)

• .agents/skills/spool/references/reading-graphs.md
• .agents/skills/select-new-goal/SKILL.md
• .agents/skills/spool-product/SKILL.md
• .agents/skills/spool-architecture/SKILL.md
• .agents/skills/spool-engineering-standards/SKILL.md
• .agents/skills/spool-release/SKILL.md

Left in place on purpose

• CHANGELOG.md historical sections, including the Unreleased stop-list. That list is the release record, and TestSunsetCopyClosesSoftDoor requires it.
• docs/context-git-migration.md — the export escape-hatch doc. Its drop list matches tool output (packs, remote config skipped as Rack remotes, reflogs, merge leases, projections). It is not the day-to-day operating skill.
• docs/graphcontract.md — library pack-contract disclaimer, not a CLI how-to.

What guidance teaches now

• Bind: spl context init --remote
• MCP: spl mcp and KEEP spl_* tools, including spl_mutate
• Writes: one mutation batch → short-lived branch + pull request
• Reads: query-context, search, search-expand, filter, resolve, graph
• schema / validate, asset *, file-graph merge *, prune
• Export: if a leftover .spl store remains, run context export / migrate-once once into the bound context git remote, then stop. Export is lossy; re-run is overwrite-at-own-risk.
• History, diff, and pull-request review: stock git and GitHub on the context remote

cmd/spl/sunset_test.go walks .agents/skills and fails if a removed verb or Rack reappears as skill text. The required .spl escape-hatch sentence is allowed. spl_context_export is allowed; the removed tool id spl_context is not.

Verification

make check passed tidy, format, vet, staticcheck, golangci-lint, go test, and go test -race.

govulncheck (v1.1.4, vuln DB 2026-10-08) reports pre-existing standard-library findings in Go 1.26.6 (fixed in 1.26.9), including GO-2026-6617. Those calls are unchanged by this PR. Quality on main last passed on 2026-09-19, before that database update. This PR does not bump the toolchain.