feat(terminal): shade tool output and show a spinner while waiting

[minixalpha]

Summary

Two terminal UX improvements to how tool activity is presented:

• Two-tone tool shading. Echoed tool calls ([bash]$ ..., [read] ...) now sit on a lighter background shade (256-color gray 238) than their output (gray 236), so the command and what it printed read apart at a glance. print_tool is split into print_tool_use / print_tool_output over a shared _print_shaded helper.
• Wait spinner. A braille-dots spinner animates in place while awaiting the model's reply or a running bash command, so long waits no longer look like a frozen terminal. A daemon thread redraws the line and erases it on stop, leaving no trace in the transcript; it draws nothing when stdout is not a TTY or NO_COLOR is set, and stop() is idempotent. The Agent> prompt is now printed only when the reply actually streams text, so tool-only turns no longer leave an empty prompt line.

Changes

• src/nanopycodeagent/terminal.py — two shading shades, Spinner class (context manager, background thread).
• src/nanopycodeagent/agent.py — use the split print helpers; wrap the model stream and bash runs in spinners; defer the Agent> prompt until the first streamed token.
• tests/test_terminal.py — cover both shades and the spinner (TTY/non-TTY, idempotent stop, erase-on-stop).
• docs/changelogs/0.6.x.md — new 0.6.x series file with the [Unreleased] entry (Added: spinner; Changed: two-tone shading), since these are features and the next release is a minor bump per SemVer.
• docs/dev_notes/zh-CN/0.6.x.md + docs/dev_notes/en/0.6.x.md — new 0.6.x dev notes (Chinese source, generated English version).

Test plan

• uv run pytest — 58 passed.