feat: add write tool for creating and overwriting files

[minixalpha]

Summary

Adds a write tool — the thin whole-file write chosen as Route A in the accompanying research — plus the research doc and dev notes that motivated it.

Research (docs/research/write_tool.md)

• Surveys the model-facing file-mutation tools of five agent projects (grok-build, pi, claude-code mirror, opencode, codex): why a dedicated write exists next to bash, what each project implements, and what none of them actually guarantee.
• Revised conclusion: a thin write is worth building now for content/command separation (it removes the class of silently corrupted heredoc writes) and for terminal presentation — not as a control plane. Staleness guards and atomic replace are deliberately out of scope in a single-threaded agent, where they would be pretend safety.

The tool (src/nanopycodeagent/write_tool.py)

• Structured {path, content} input: content lands on disk as data — no shell expansion, heredoc delimiters, or quote escaping.
• Creates the file (missing parent directories included) or replaces it whole; the result states which happened, with byte and line counts.
• Path handling matches read: absolute or relative to the working directory, leading ~ expanded.
• Refuses directories and non-regular files — a FIFO write-open would block with no timeout, a device file must not be clobbered as text; a symlink to a regular file writes its target.
• Content is written exactly as given (UTF-8, no newline normalization); a lone surrogate is an error instead of a silently corrupted file.
• Every error states the next step, read-style — e.g. a quoted ls -ld -- <parent> bash hint when the write fails, or bash as the escape hatch for a FIFO target.
• The description is honest: plain overwrite, last writer wins, no prior-read or staleness checks claimed.

Wiring and display (src/nanopycodeagent/agent.py)

• Registered in TOOLS and the system prompt. The terminal echoes [write] <path> plus the content folded to its first 10 lines (over-long lines cut at 200 chars) on the tool-use shade, with the result on the output shade.

Tests

• 17 new cases covering run_write and content_preview (create/overwrite, parent creation, CRLF passthrough, FIFO rejection, symlink target, surrogate error, preview folding) plus an agent-loop dispatch test.

Docs

• Changelog entry under [Unreleased] in docs/changelogs/0.6.x.md.
• Dev notes: the write-tool plan added to the Chinese source, English version regenerated from it.

[minixalpha]

Merged into main as 11bb0b7 — the merge API call was interrupted by a 502 upstream, which updated main but left this PR record open. Closing manually; all commits are contained in main.