Fix DAG arrow-key nav stolen by Details roster scroll

[anutron]

Summary

• Reported bug: in the native Hera coordinator view, pressing Up/Down/j/k to navigate the embedded plan-DAG graph instead scrolled the "Agents" roster shown above it in the Details region — the roster claimed those keys FIRST whenever it had unseen rows (a prior "layered, not duplicated" design), only falling through to the plan widget once the roster hit its scroll bound.
• Fix: split the keysets. internal/tui/hera/page.go's rosterScrollDelta/handleDetailsKey now route PgUp/PgDn exclusively to DetailsView.ScrollRoster, and j/k/Up/Down/h/l/Enter/Space/Esc reach planview.Widget.InputHandler unconditionally — the two surfaces can no longer contend for the same keystroke.
• No change to rail-focused navigation (Up/Down/j/k/s/S/m/M etc. on the left rail) or to any other focus-ladder behavior (Tab/Ctrl+Alt+arrows/Ctrl+Q/Ctrl+Z) — this only touches the Details/plan region's own key routing.
• Docs: added a context/knowledge/gotchas/hera-view.md bullet + updated the README's plan-DAG key table (PgUp/PgDn row added, arrow-key row corrected).

Test plan

• New regression tests in internal/tui/hera/dag_test.go: TestFocusRouting_ArrowKeysStayWithinFocusedRegion (end-to-end SimulationScreen test proving rail-focused keys are unaffected and DAG-focused arrows never leak to the rail cursor or roster scroll), TestHandleDetailsKey_ArrowsAlwaysReachPlanNeverRoster, TestHandleDetailsKey_PgDnPgUpScrollRoster.
• Updated TestRosterScrollDelta (details_test.go) for the new PgUp/PgDn keyset.
• make pre-pr clean (build, vet, fmt-check, lint-pr, test-cover-gate all pass; vuln fails only on pre-existing advisory stdlib CVEs, confirmed toolchain-only and CI-continue-on-error, unrelated to this change).
• Full internal/tui/... suite green.

🤖 Generated with Claude Code

Co-Authored-By: Claude noreply@anthropic.com

[github-actions]

Merging this branch will decrease overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/drn/argus/internal/tui/hera	92.29% (-0.01%)	👎

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Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/drn/argus/internal/tui/hera/details.go	99.63% (ø)	269	268	1
github.com/drn/argus/internal/tui/hera/page.go	88.00% (-0.07%)	350 (-2)	308 (-2)	42	👎

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.

Changed unit test files

• github.com/drn/argus/internal/tui/hera/dag_test.go
• github.com/drn/argus/internal/tui/hera/details_test.go