docs(conventions): amend ADR 0075 for evidence-gated plugin trigger lines and add the setup-lifecycle owner doc #9801

Refs #6594. Slice 1 of 4. Runs first; slices 2 and 4 wait on it.

Problem

The owner decided on 2026-10-24 (Q98, comment on #6594) that a plugin's setup may add one trigger line to the always-loaded root AGENTS.md, but only with stumble evidence that the plugin needs it. On origin/main two records still forbid it:

• docs/adr/0075-resolve-plugin-settings-through-one-four-layer-cascade-and-a-bound-convention-home.md:96-97: "A setup action offers to write the convention files, and nothing is always-loaded."
• docs/conventions/config-cascade/README.md:297: "nothing is always-loaded".

developer-experience already ships such a line inside a <!-- BEGIN GENERATED: plugin-conventions --> block (plugins/developer-experience/skills/setup/scripts/pointer-line.sh:23-26), so the shipped behavior is undocumented. No owner doc for the setup lifecycle exists: docs/conventions/setup-lifecycle/ is absent on origin/main.

Fix

1. Amend ADR 0075 in place, following the repository's amendment precedent (status line accepted; amended 2026-10-24 (...) plus an ## Amendment (2026-10-24): ... section, as in docs/adr/0062-*.md:3 and its ## Amendment (2026-10-16) heading). The amendment allows one trigger line per plugin inside the shared GENERATED: plugin-conventions block of the root AGENTS.md, written by that plugin's setup after confirmation, naming only model-invokable skills, and allowed only with that plugin's own stumble evidence (docs/plugin-philosophy.md Instruction economy). It keeps the developer-experience block as the shipped form.
2. Write docs/conventions/setup-lifecycle/README.md as the owner doc: the managed block and its one-line-per-plugin ownership rule (from pointer-line.sh), the evidence gate, the check/apply verb set (pointing at docs/plugin-philosophy.md setup section, not restating it), and placeholders that name slices 2 to 4 (provenance, converge-by-state apply, current-facts check) as the sections they fill. No user-level ~/.config/<plugin>/ layer: ADR 0075 fix 9 retired the user-global file layer into userConfig.
3. Add a row for it to the Convention registry in docs/plugin-philosophy.md (line 822 onward), and amend docs/conventions/config-cascade/README.md:297 to point at the new rule.

Out of scope: migrating existing .claude/* surfaces (#5906), the resolver and setup writer already delivered under ADR 0064 and #7934, #7938, #7939, and editing this repository's own AGENTS.md.

Done when

• test -f docs/conventions/setup-lifecycle/README.md exits 0
• grep -c 'GENERATED: plugin-conventions' docs/conventions/setup-lifecycle/README.md prints 1 or more
• grep -c '^## Amendment (2026-10-24)' docs/adr/0075-*.md prints 1
• grep -c 'nothing is always-loaded' docs/conventions/config-cascade/README.md prints 0
• sed -n 3p docs/adr/0075-*.md contains amended 2026-10-24
• grep -c 'setup-lifecycle/README.md' docs/plugin-philosophy.md prints 1 or more
• bash scripts/check-conformance-registry.sh --check and bash scripts/check-adr-numbers.sh --check exit 0
• ci-status is green on the PR head