feat(workstation-tabs): retire the Files tab when a file opens

[Chloe-JY]

Problem

The Explorer ("Files") tab is a transient picker: it exists so the user can browse the tree and open a file. But after a file was opened, the Files tab stayed in the strip next to the new file tab. Every browse-then-open flow left a stale Files tab behind that the user had to close by hand.

Solution

openTab in src/store/workstation/tabs/tabMutations.ts now retires an open Explorer tab whenever a file tab is opened:

• A newly created file tab is spliced into the Explorer's slot, so the strip position is preserved (a genuine replace, not close-then-append).
• Re-activating an already-open file also retires the Explorer tab, so clicking an open file from the tree behaves identically.
• Non-file tabs (search, terminal, source-control, browser, directory listings) leave the Explorer alone.
• A pinned or closable: false Explorer is treated as a protected fixture and never retired (currently unused — usePinnedTabs is wired enabled: false — but keeps the fixture contract).

The fix lives in the single pure mutation that every open path funnels through (file-tree click, chat path references, spotlight, EditorTabService, agent-driven opens), rather than being patched per call site.

Potential risks

• Behavior change applies in every station mode, not just my-station — the tab strip is a single shared pool, and gating a pure mutation on stationModeAtom would thread store state into it. Scoping it per mode is possible as a follow-up if wanted.
• Once a file is open, ⌘G focuses the last file rather than reopening Files (pre-existing EditorTabService.getLastFileOrExplorerTabId preference); Files stays reachable via the Launchpad / + menu, or ⌘G with no file tabs open.
• The Explorer tab is workspace-local, so removal flows through the normal splitPanel persistence with no shared-resource teardown; no migration concerns.
• Tab lifecycle only, no visual restyle — screenshots would show an unchanged tab strip, so none are attached. The change was not exercised in the running Tauri app.

Verification

Verified in a detached worktree at origin/develop (57c8ffd) containing only this PR's two files (node_modules symlinked), so the diff compiles and passes against the base alone:

• npx tsc --noEmit --pretty false -p tsconfig.json — exit 0
• npx vitest run --config config/vitest.config.ts src/store/workstation/tabs/__tests__/tabMutations.test.ts — 19 tests pass (4 new: replace-in-place, reopen-retires, non-file untouched, pinned protected)
• npx eslint over both files — clean

On the dev checkout, the broader sweep vitest src/store/workstation src/hooks/tabHost src/modules/WorkStation src/services/workStation — 144 files / 1050 tests pass.

Not run: E2E; manual check in the running app. The commit was built with git plumbing from a live shared checkout, so git hooks did not run and the Pre-commit hook ran. trailer is absent — typecheck/lint/tests above were run manually instead.