Tasks: a Slack app from one click, named by its operator

Task list

[x] A1: red tests. Naming, formats, link and kept-file tests for channels manifest, failing before the change. Req: R1–R3 · Test: T1–T3.
[x] A2: app_manifest module. handle, check_name, build, link, render_json, default_path, delta, write. Req: R1–R3 · Test: T1–T3. Deps: A1.
[x] A3: the verb's options. --name, --format, --link, --write [PATH] and the report. Req: R1–R3 · Test: T1–T3. Deps: A2.
[x] B1: walkthroughs. /the-loop:init step 5, /the-loop:upgrade-the-loop's new step, .the-loop/manifest.yaml. Req: R4, R5. Deps: A3.
[x] C1: docs and evidence. The packaged manifest's header and the Slack guide, the onboarding guide, the CLI reference, docs/capabilities/channels.md, and the evidence files. Deps: A3, B1.