refactor: unify TUI tabs behind an entry interface; fix convention violations

[lvlcn-t]

Why

The CRUD series left a type-tag dispatch smell: four near-identical tab
types (browseTab, TenantsTab, ContextsTab, CredentialsTab) fed by a
12-method tabs.Manager interface no test ever mocked, with Tabs
re-discriminating the selected item's concrete kind in four separate
type-switches. A generic details.Item routed through a switch on its
kind is neither polymorphism nor concrete typing.

What

Real polymorphism, driven by the item types:

• entry interface (embeds list.Item + details.Item, adds
  name/blank/form/save/remove) — each item (TenantItem,
  ContextItem, CredentialItem) carries its own CRUD behaviour as
  methods, with compile-time var _ entry assertions. save switches on
  the submission intent internally, on the item.
• activatable — the context-only "use" side-effect, type-asserted
  once (like io.WriterTo), replacing handleInteractiveSelect.
• One Tab struct replaces the four tab types. Tabs holds a concrete
  *contexts.Manager (tabs.Manager interface deleted) and dispatches
  through entry methods with zero type-switches.

Net -219 lines.

Convention fixes (from the audit)

• contexts: RenameContext/DeleteContext and the tenant/credential
  delete path wrap the existing ErrContext*/ErrTenant*/ErrCredential*
  sentinels with %w; renameAndRetarget takes the renameTarget struct
  instead of 5 params; entryTarget.deleteX → remove.
• form.Model stores the last validation error (not its text), exposes
  Err(); form tests assert errors.Is.
• tui/tabs tests drop every assert.Contains on View()/status/
  err.Error(). Pure validators are unit-tested with errors.Is/
  require.Error; duplicate-create rejection asserts
  contexts.ErrTenantExists via a lastErr accessor.
• Domain not-found error tests folded into a table using errors.Is.

Verification

• go test -race ./..., go vet, full pre-commit — green.
• Invariants grep-verified: no tabs.Manager interface; no concrete-item
  type-switch outside item files; no message-substring assertions;
  contexts still a leaf; tui never imports cmd.
• CLI smoke test unaffected; behaviour preserved by the existing CRUD flow
  tests.

Based on feat/tui-crud-credentials (#23); reviewing the stack #20→#23
first is recommended.

[github-actions]

File	Coverage
All files	62%	❌
az/azconfig.go	66%	❌
az/client.go	0%	❌
az/client.go	0%	❌
az/commands.go	0%	❌
az/secrets.go	0%	❌
az/secrets.go	15%	❌
cmd/context_view.go	85%	✅
cmd/current.go	80%	✅
cmd/delete_context.go	71%	✅
cmd/get.go	56%	❌
cmd/list.go	46%	❌
cmd/output_flags.go	85%	✅
cmd/rename_context.go	69%	❌
cmd/set_context.go	69%	❌
cmd/set_credential.go	97%	✅
cmd/set_credential.go	80%	✅
cmd/set_tenant.go	95%	✅
cmd/set_tenant.go	57%	❌
cmd/use.go	47%	❌
cmd/view.go	95%	✅
cmd/view.go	48%	❌
config/config.go	91%	✅
config/credential.go	0%	❌
config/credential.go	64%	❌
config/credential.go	70%	✅
config/keyvault.go	14%	❌
config/loader.go	82%	✅
config/loader.go	79%	✅
config/store.go	69%	❌
config/writer.go	69%	❌
contexts/contexts.go	92%	✅
contexts/contexts.go	82%	✅
internal/gendemo/main.go	0%	❌
internal/gendoc/main.go	0%	❌
keyvault/azure.go	0%	❌
keyvault/azure.go	0%	❌
keyvault/keyvault.go	75%	✅
login/login.go	76%	✅
main.go	0%	❌
output/printer.go	79%	✅
semver/version.go	94%	✅
tui/app.go	0%	❌
tui/app.go	0%	❌
tui/tui.go	0%	❌
tui/details/viewer.go	0%	❌
tui/form/form.go	67%	❌
tui/keys/builder.go	80%	✅
tui/keys/keys.go	66%	❌
tui/splash/splash.go	0%	❌
tui/splash/splash.go	0%	❌
tui/state/state.go	71%	✅
tui/styles/azure_renderer.go	33%	❌
tui/styles/azure_renderer.go	8%	❌
tui/tabs/confirm.go	36%	❌
tui/tabs/context_item.go	59%	❌
tui/tabs/credential_items.go	92%	✅
tui/tabs/credential_items.go	35%	❌
tui/tabs/list_builder.go	83%	✅
tui/tabs/tab.go	96%	✅
tui/tabs/tab.go	53%	❌
tui/tabs/tabs.go	51%	❌
tui/tabs/tenant_items.go	67%	❌
wif/factory/factory.go	0%	❌
wif/oauth2/browser.go	0%	❌
wif/oauth2/browser_linux.go	0%	❌
wif/oauth2/cache.go	76%	✅
wif/oauth2/cache.go	79%	✅
wif/oauth2/oauth2.go	25%	❌
wif/oauth2/oauth2.go	59%	❌
wif/options.go	20%	❌

Minimum allowed coverage is 70%

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