docs: correct the sizing-aware lists and the han-reporting install claims

[mxriverlynn]

Closes #200.

An outside reader worked through the docs end to end and found pages that disagree with each other and with the skill definitions. All three of their findings were verified against the skill frontmatter and the plugin manifests before anything was changed, and the first one turned out larger than reported.

The sizing-aware lists were three different lengths

docs/quickstart.md listed 9 skills and docs/concepts.md listed 12. Thirteen skills classify their own work, which is what docs/sizing.md already said. Both pages now match it.

The reporter found one missing skill. Four were missing, and the cause is one line: CONTRIBUTING.md step 6 told contributors to update the sizing and concepts catalogs and never mentioned the quickstart. Four skill additions between 2026-08-10 and 2026-09-09 updated the two named pages and left the third behind, so the quickstart's list had been stale since 2026-05-29. Step 6 now names all three.

plan-a-feature-to-confluence declares arguments: size and stays out of all three catalogs on purpose. It forwards the argument to plan-a-feature rather than classifying anything, which step 6's own opening condition already excludes.

Two pages denied the han-reporting exception they document

han-reporting is the only layer plugin that does not declare han-core, and both pages said so correctly before contradicting themselves.

On docs/choosing-a-han-plugin.md the correct half sat in a trailing parenthetical while the bold sentence two lines below it, under the heading "The one thing that surprises people", generalized over it. A skimming reader stops on the bold sentence, so the exception now lives there.

docs/concepts.md listed reporting-only among installs that do not exist. It is exactly the install that does exist, so the page now says what it gives you: /stakeholder-summary and /html-summary plus han-communication, without the han-core agent roster.

The banner has alt text

README.md carries the only image embed in the repo's prose and had no alt attribute, so the project name and tagline reached a screen-reader user as the filename or as nothing.

What is in this PR

Two commits. The first adds the change plan, the decisions behind it, and the evidence, under docs/plans/gh-200-docs-consistency/. The second applies the six edits.

Each edit was pinned to exact replacement text before being made, and each of the plan's three verification checks passes: all three pages now carry the same 13 names as docs/sizing.md, no page outside docs/plans/ claims that every layer install brings the shared agents, and the banner announces the project rather than its filename. npm run lint and npm test pass.

Deliberately not in this PR

An automated check that a skill declaring arguments: size reaches every catalog. It would have caught this drift on the day it started, where the checklist edit only reminds a person. It is deferred with a reopening trigger recorded in the plan: a second drift incident, or a fourth page carrying the list.

Two things the review surfaced and this PR leaves alone, both noted in the plan's Open Items. The plugin dependency graph is hand-copied across four files with nothing enforcing agreement, which is the same failure mode on a different fact. And han-reporting/README.md states no dependencies at all.