feat(harness): SourcePicker Add Include context (Phase 3)

[eyelock]

Summary

Phase 3 of the unified source-picker plan, completing the picker abstraction across the three "add a thing" surfaces:

• Phase 1 (feat(harness): unified SourcePicker for Install Harness (Phase 1) #262) — Install Harness on the new SourcePicker
• Phase 2 (feat(harness): SourcePicker Add Delegate context (Phase 2) #267) — Add Delegate
• Phase 3 (this PR) — Add Include

The bespoke 790-line AddIncludeFlow retires in favour of AddIncludeContext driving the same SourcePicker shell as the other two contexts.

Changes

New files

• Sources/TermQ/Views/SourcePicker/AddIncludeContext.swift — SourcePickerContext for adding includes. Library tab lists plugins from configured marketplaces with search and "already installed" recognition; picking a fresh plugin enters a Configure stage with the picks tree (IncludePicksSelector) and a command preview. Git URL tab takes URL/ref/path with the same Apply path. Inherits the post-pick Configure stage shape from Add Delegate.
• Sources/TermQ/Views/SourcePicker/IncludePicksSelector.swift — moved from AddIncludeFlow.swift; reusable view kept for the Configure stage (and the existing Edit Include sheet which already uses it).
• Sources/TermQ/Marketplace/IncludeMatching.swift — IncludeKey and IncludePluginLookup non-View types moved here from the old flow file. Used by both the new context's already-installed detection and the existing Edit sheet.

Modified

• Sources/TermQ/Views/HarnessDetailDependencyView.swift — drops the inline AddIncludeSectionView; "+ Add Include…" button now triggers editor.startAddingInclude() which the IncludeEditorOverlay presents as a sheet (mirrors AddDelegateSheetHost).
• Sources/TermQ/Views/HarnessIncludeEditor.swift — isAddingInclude semantics shift from "show inline panel" to "show sheet"; existing didFinishAddingInclude(harnessName:) flips the flag and reloads detail.
• Sources/TermQ/Utilities/Strings+Sidebar.swift — adds harnesses.include.add.edit.existing; retires step-title and source-mode strings (no longer needed — picker shell handles tab labels and step dots are dropped).

Deleted

• Sources/TermQ/Views/AddIncludeFlow.swift (790 lines).

Tests

• Tests/TermQTests/AddIncludeStoreTests.swift → renamed to AddIncludeContextTests.swift, rewritten against the new context API (15 tests covering library-stage resolution, picks helpers, command-preview behaviour for Library and Git URL paths).

Localization

• 6 retired strings deleted from all 40 locales (harnesses.include.add.step.*, harnesses.include.add.source.*).
• 1 new string (harnesses.include.add.edit.existing = "Edit") added across all locales.

UX changes from the prior inline flow

• Sheet replaces inline expansion. Same step-machine inside; just a different container. Matches Install Harness and Add Delegate.
• "Already added" detection unchanged — clicking an already-installed plugin row routes to the existing Edit Include sheet via editor.switchToEditing(match). Critical UX preserved.
• Step dots dropped (Add Delegate has no dots either; consistent now).
• Sheet size 520×620 to fit the Configure stage cleanly without cutting off the Cancel button.
• "None" button correctly disables Apply (previously left Apply enabled, which would have resulted in a no-op include with --pick defaulting to all artifacts).
• Command-preview Apply path emits AddInclude: log lines for diagnostic forensics.

Test plan

• Open Add Include on an editable harness; Library tab shows plugins from configured marketplaces with search filter.
• Pick a fresh plugin → Configure stage with picks tree and command preview.
• Pick a plugin already on the harness → "Edit" button instead of "Pick"; click routes to Edit Include sheet.
• Apply on the Library path → sheet dismisses; detail view immediately shows the new include with correct picks.
• Apply with all picks selected → command preview omits --pick.
• Apply with subset of picks selected → command preview includes --pick.
• Apply with no picks selected (after using the "None" button) → Apply button is disabled.
• Lazy-load picks: pick a plugin with skillsState != .eager; spinner shows; picks populate.
• Git URL tab: URL + ref + path → Apply works; no picks step.
• Cancel and Esc both dismiss the picker.
• Install Harness sheet (feat(harness): unified SourcePicker for Install Harness (Phase 1) #262) and Add Delegate sheet (feat(harness): SourcePicker Add Delegate context (Phase 2) #267) still work — no regressions.

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