e2e: subagent-child-navigation 'not found fallback' fails with Close Tab strict-mode violation on all CI platforms

[alltomatos]

`e2e/regression/subagent-child-navigation.spec.ts`'s "shows the not found fallback when the viewed session is deleted" fails consistently on `unit (windows)`/`e2e (windows)` and `e2e (linux)` CI jobs (confirmed identically failing on `dev` HEAD baseline, and on unrelated PRs #190/#191 that don't touch this file), always at the same assertion:

```
Error: strict mode violation: getByRole('button', { name: 'Close Tab' }) resolved to 2 elements:
1) <button ... aria-label="Close tab" ...> aka getByRole('button', { name: 'Close tab', exact: true })
2) <button ... data-icon="xmark-small" ...> aka getByRole('button', { name: 'Close Tab', exact: true })

### Root cause

Two different i18n keys produce near-identical accessible names:
- \`common.closeTab\` → "Close tab" (the tab strip's own close-tab icon button, always present when a tab is open)
- \`session.error.notFound.closeTab\` → "Close Tab" (the session-not-found fallback's own button)

Playwright's \`getByRole\` name matching is case-insensitive by default, so \`page.getByRole("button", { name: "Close Tab" })\` (no \`exact\`) matches both whenever they're both on screen — which is this test's exact scenario (a tab open, showing the not-found fallback inside it). This is not timing-dependent flakiness; it's a deterministic locator ambiguity in the test, which is why it fails identically on every platform/run.

### Fix

#195 — adds \`exact: true\` to the locator, making the match case-sensitive so it resolves only the fallback's own "Close Tab" button.