docs: rewrite the README as a landing page, extract the manual into docs/ (#92)

[2000game]

Closes #92. Stacked on #93 (the Handbuch split) — merge that first.

The README was 572 lines. It read as a manual, not a front page: the environments story, the entire CI section, the auth handshake and every per-feature detail lived inline, and the actual hook — a config plus the plan it produces — sat around line 140.

After

~210 lines, in the order a newcomer reads:

1. What it is — structure-as-code, plan/apply, people never managed
2. Why — a "clicking vs ct" table (audit trail, repeatability, rehearsal env, drift, preview)
3. Show me — a zero-id config and the ct plan output it produces
4. Install → first run
5. What it manages — one table, each row linking to its guide
6. Environments and CI — four commands, then links
7. Guardrails, two-repo model, docs index, development, status

The plan output in "Show me" is real renderPlan output, produced by running the renderer over a plan object — not hand-written to look plausible.

Nothing deleted, only relocated

From README	To
Environments, protected envs, dev→prod promotion	docs/environments.md (new)
CI auth, the auth handshake, --detailed-exitcode, --json shape, drift-vs-config attribution, the PR-comment job	docs/ci.md (new)
Portable references, campus assignment, allowDuplicateName, stdout/stderr convention	docs/configuration.md (new)
The phase-by-phase status list	Deleted — it duplicated #1 and #19, which are linked instead

Verification

• Every relative link in the README resolves to a file that exists (checked mechanically).
• Every DSL name in the "What it manages" table checked against ConfigContext — including ct.ageGroup / ct.targetGroup / ct.relationshipType, which the old README never mentioned.
• npm run lint, npm run typecheck, npm test (570 passed, 5 skipped) → green.

Both claims that had to stay above the fold did: people are never managed, and only explicitly declared or adopted resources are ever touched.

https://claude.ai/code/session_01KPxjSEkqMiU7WE3vg6C3am