refactor(cadence): drop dead cadence block from people/edpa configs (cadence is iteration-sourced)

[jurby]

Why

The cadence: block in the people.yaml template (iteration_weeks / pi_weeks / delivery_iterations_per_pi / ip_iterations_per_pi) is dead config — nothing in the engine reads it. Effective cadence is derived per Planning Interval from the pi: block of .edpa/iterations/PI-*.yaml (_pi_loader.derive_pis), or inferred from each iteration's weeks:/dates. The block had also been copied into edpa.yaml examples and e2e fixtures with a misleading "must mirror" note (no code ever verified the match), so it invited "edit this to change the rhythm" mistakes.

This removes the dead artifact everywhere and points users at the real surface.

What changed

• Templates: drop cadence: from people.yaml.tmpl; add a breadcrumb to the iteration pi: block. people.yaml is now just teams + people.
• Code (doc-drift): mcp_server.py edpa://config resource description (it serves edpa.yaml but claimed "PI, iterations, cadence, sync settings"); backlog.py "(people, teams, cadence)" comment (cadence is loaded into the dict but never read).
• Examples/fixtures: docs/kashealth-pilot/{edpa,people}.yaml.example, docs/examples/capacity-small-team.yaml, tests/e2e_v2_full/fixtures/{edpa,people}.yaml (incl. the stale "Wave C verifies this match" comment).
• Docs/web: remove the cadence: snippet from README / SETUP / quick-start / playbook (CZ+EN) and the web setup wizard's generated config; docs/cadence.md gains a "Where cadence is configured" section. Conceptual 2/10-vs-1/5 content is unchanged.
• Release: plugin 2.1.9 → 2.1.10 + CHANGELOG entry.

Verification

• pytest: 577 passed.
• npm run build (web): 28 pages built OK.
• grep: no pi_weeks / delivery_iterations_per_pi / ip_iterations_per_pi outside CHANGELOG.md / docs/proposals/.
• engine smoke (backlog.py tree) loads people.yaml fine.

No engine behaviour change — cadence was already iteration-sourced.

Follow-up (out of scope here)

• The web setup wizard (setup.astro CZ+EN) generates a non-canonical config (edpa: / scoring: / outputs: shape doesn't match the real v2 edpa.yaml / people.yaml / cw_heuristics.yaml split). Only the dead cadence: block was removed from its output; the generator needs a separate overhaul.
• A full 2.1.10 release should also bump web/package.json + lockfile and doc version references (left to the release step).

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