feat: pin backend/model defaults with --default and default-aware pickers

[nelopuchades]

Motivation

Small Harness reads backend / modelOverride from agent.config.json at
launch, but the only way to change those project defaults was to re-run /setup
or hand-edit the file. Switching model or backend mid-session (/model,
/backend) applied for that session only and silently reverted on the next
launch, with no indication in the picker of what was actually persisted.

This change lets you pin the active — or newly selected — backend/model straight
into agent.config.json, and makes the interactive pickers show what is live
versus what is saved on disk.

Summary of changes

Persisting defaults

• Add /model --default and /backend --default to surgically merge backend
  and modelOverride into agent.config.json without re-running /setup
• /model … --default writes backend + modelOverride; /backend … --default
  writes backend and clears modelOverride so the next launch uses that
  backend's built-in default model
• Session changes still apply if the file can't be written; invalid or non-object
  JSON is never overwritten

Default-aware pickers

• Tag each /model and /backend entry (selected) when it is the live session
  choice and (default) when it matches what would load on next launch (read via
  the new config::persisted_defaults helper); both show when they coincide
• Append --default to a picker selection (e.g. 3 --default) to pin it as you
  choose; otherwise a y/N prompt offers to save the choice as the project default

Docs

• Document the flag, the inline --default pinning, and the markers in the README
  and Unreleased CHANGELOG

Verified by

• cargo fmt --all -- --check
• cargo clippy --all-targets -- -D warnings
• cargo test (565 passed)

Screenshots