Add Junie as an ACP provider

[preigile]

Human comments

Support for the Junie agent has been added via the ACP

What was wrong

bb had no registration for JetBrains' Junie

What changed

• packages/provider-bridge-acp/src/wire.ts: SessionConfigSelectOptions is parsed as the union the ACP spec defines, and groups are flattened to their values (bb has no grouped picker). The lenient path for non-model options flattens groups the same way. Any ACP agent that groups its options benefits, not only Junie.
• packages/provider-bridge-acp/src/bridge/provider-maintenance.ts: loginCommand, readAccount and readUsage are optional. Health reports an installed agent whose credentials bb cannot read as ready instead of guessing "signed out", and usage without readUsage is supported: false. Added JUNIE_ACP_MAINTENANCE – installer only, JetBrains' documented script – and packages/provider-bridge-acp/src/dialect.ts gains the junie dialect that carries it (Junie reads no vendor side channel; the dialect exists only for that).
• plugins/provider-acp: the acp-junie agent (junie --acp true, .junie/skills and .agents/skills native roots, fork: "none" until Junie's initialize reply is verified), its brand icon, and providerInstallation so bb installs the CLI itself the same shape as acp-cursor. The agent is listed whether or not the CLI is present, because an installed-only listing hides the row the Install action lives on. Reserving an id is now an explicit reserved flag instead of a side effect of always being listed, so acp-junie stays overridable via customAgents while acp-cursor does not.
• apps/web: the Junie mark in the "Works with" row and in the two provider lists on the landing page.
• User-facing surfaces updated in the same change: docs/configuration.md, the bb providers guide template, the bb-cli skill, the bb-app README.
  No host-daemon wire change: the dialect id travels in the existing providerOptions, so HOST_DAEMON_PROTOCOL_VERSION is unchanged.

How you verified

• Tests added
• Commands run (turbo typecheck/test filters)
• Manual checks.