Align admin routing docs and tab-path conventions with implementation

[mdon]

• Rewrite custom-admin-pages.md with explicit anti-pattern section covering layout loss and cross-live_session navigation failure modes
• Correct claim that dynamic segments require a route module; tab_to_route/1 splices paths verbatim, so live_view: on a tab supports :id/:slug via visible: false hidden tabs (as real modules like posts/catalogue do)
• Correct claim that admin_routes/0 accepts controller/forward/scope; the quoted block is spliced inside live_session :phoenix_kit_admin which only permits live declarations — redirect non-LiveView routes to generate/1 or public_routes/1 with phoenix_kit_sync and phoenix_kit_publishing as refs
• Fix Mix task name (phoenix_kit.gen.admin.page) and argument shape in Igniter generator section to match actual parse_args/2 in the task
• Fix LayoutWrapper attr name from url_path to current_path in AGENTS.md and remove non-existent current_locale_base example
• Convert tab-path examples to the relative-form convention used by every real plugin across ADMIN_README.md, dashboard/README.md, tab.ex moduledoc, dashboard.ex, registry.ex, config.ex, tabs_initializer.ex, and the two dev_docs guides
• Convert the built-in jobs tab from absolute to relative path form for consistency with the rest of the ecosystem
• Add historical-exploration banner to 2025-12-30 routing-architecture guide so readers know Strategy 1 was not adopted
• Add cross-session caveat note to PHOENIXKIT_AWS_COMPATIBILITY.md explaining the full-page-reload limitation of the override pattern
• Fix Igniter generator After Generation section to stop teaching a separate-live_session pattern that recreates the known bug

Motivation: these docs were drifting ahead of the implementation and would have led module authors into the exact "parent-router hand-registration" bug class (layout loss + cross-live_session navigation crash) that we just finished warning against in every other file. Every claim here is now traceable to a file:line ref in integration.ex, auth.ex, tab.ex, or layout_wrapper.ex.