Isolate balade's Pi credentials and model preference from a user's own Pi installation

[PhilDL]

Problem

balade generate currently shares state with a user's own Pi installation:

• Credentials — ModelRuntime.create() uses Pi's default auth store, ~/.pi/agent/auth.json. Logins made through balade land in the user's Pi credential file, and balade reuses logins the user made with the pi CLI.
• Default model — every explicit or confirmed selection calls setDefaultModelAndProvider, which Pi writes to the global ~/.pi/agent/settings.json. A balade run therefore silently changes the default provider/model of someone's own pi CLI.

The authoring session itself is already isolated (SessionManager.inMemory, SettingsManager.inMemory, no extensions/skills/themes) — this issue is only about the outer account/preference layer in src/generate/pi.ts.

This revisits a recorded trade-off: DECISIONS.md ("Provider/model defaults use Pi's global SettingsManager rather than a second balade preference file"). The decision entry must be updated with the outcome.

What to build

Stop mutating a Pi user's configuration. Decide and implement one of:

1. Fully separate agent dir — point Pi at a balade-owned directory (SettingsManager.create(cwd, agentDir), ModelRuntime.create({ authPath }); Pi also honors PI_CODING_AGENT_DIR). Cleanest isolation; the cost is that a user already logged into Pi must authenticate again for balade.
2. Shared auth, own preferences — keep reading/writing ~/.pi/agent/auth.json so existing Pi logins keep working, but store balade's remembered model in a balade-owned settings location instead of Pi's global settings.json. Preserves login reuse; balade never rewrites Pi defaults.

Either way, balade's own state should live in a predictable, documented location.

Acceptance criteria

• Running balade generate and picking a model never changes the default provider/model of a standalone pi CLI installation.
• The chosen credential-storage behavior (shared vs. separate) is deliberate, documented in the README, and recorded in DECISIONS.md as the revision of the existing entry.
• Preference read/write failures remain typed warnings that do not block a generation run.
• Adapter tests cover the storage seam through injected stores, per the testing rules.

Blocked by

None — can start immediately.