bug(tui): key routing defects — Enter swallowed by overlays (sessions/plugins/dashboard), bare slash dispatch, ctrl+c draft loss

[dennisonbertram]

Found by a live tmux-driven TUI test sweep (fake provider rigs). Evidence: .tmp/tui-test/results/ (history.md, runs.md, workspace.md, pickers.md, inputux.md).

Root cause for the headline bugs: the global Submit key case (cmd/harnesscli/tui/model.go:2884) precedes overlay routing (model.go:3301/3243) and lacks branches for several overlays, so Enter falls through to the input area.

Bugs (acceptance criteria — each fixed with a regression test)

• /sessions Enter never selects a session — session switching impossible in the real TUI (model.go:2884 vs :3301).
• /plugins Enter enable/disable toggle unreachable (model.go:3243/3256).
• /dashboard n/s (new-run / steer) inputs can never be submitted; spaces are also dropped in those inputs (tea.KeySpace unhandled) — "go faster" becomes "gofaster".
• /model Enter on an empty filter result hijacks into the /keys overlay (model.go:3006 ignores Accept()'s ok=false).
• Bare / + Enter executes /add-dir — dropdown auto-accepts the first suggestion and the immediate-execute path runs it.
• Bare /continue + Enter is swallowed — aliases not in the suggestion list; the no-suggestion arm (~model.go:3067) only handles unregistered names.
• ctrl+o "expand" is inverted on completed cards (full output unreachable despite the hint); plan-mode arm of ctrl+o dead after the first tool call (activeToolCallID never cleared).
• Ctrl+C with a non-empty draft quits the TUI, losing the draft — Quit case (model.go:2610) intercepts before inputarea's clear-buffer handler (inputarea/model.go:170).
• /config search advertised (/ search... header) but dead — keys never routed to the config panel; values hard-truncate at 20 chars with no ellipsis.
• /model search drops batched/pasted multi-rune input — HandleSearchKey only accepts single-rune keys.

Test plan

Failing Go regression tests first per bug (pattern-match slashcomplete_dropdown_test.go, overlay_wire_test.go, dashboard_test.go), then minimal fixes; scripts/test-regression.sh green.

Rollout/rollback

TUI-only changes in cmd/harnesscli/tui/; rollback by reverting the branch.