feat(tui): contexts CRUD end-to-end

[lvlcn-t]

What

PR 3 of the TUI CRUD series (stacked on #21). Adds create, edit,
rename, and delete for contexts.

Note

Based on feat/tui-crud-tenants (#21). Review #20 and #21 first.

Changes

• ContextsTab binds n/e/r/ctrl+d and emits the matching actions.
• contextForm collects name, tenant, credential, and subscription.
  Tenant and credential are validated against existing entries (their
  names shown as placeholders); on edit the name is locked read-only.
• Dispatch maps intent to CreateContext/UpdateContext/RenameContext/
  DeleteContext. Edit updates in place; deleting the active context
  warns in the status line (DeleteResult.WasActive).
• Extracted shared form-field key constants; entryName() reads an item's
  config name without the list display marker (contexts render a ●/○
  marker in their title).

Testing

• Context flow tests: create (incl. unknown-tenant validation rejection),
  in-place edit, rename, delete.
• Full go test -race ./... and pre-commit green; tui never imports
  cmd.

Next

Credentials (type-driven form) in PR 4.

[github-actions]

File	Coverage
All files	61%	❌
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	0%	❌
config/credential.go	64%	❌
config/credential.go	48%	❌
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	93%	✅
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/browse.go	56%	❌
tui/tabs/confirm.go	36%	❌
tui/tabs/context_item.go	0%	❌
tui/tabs/contexts.go	49%	❌
tui/tabs/credential_items.go	0%	❌
tui/tabs/list_builder.go	83%	✅
tui/tabs/tab.go	70%	✅
tui/tabs/tabs.go	88%	✅
tui/tabs/tabs.go	54%	❌
tui/tabs/tenant_items.go	0%	❌
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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