fix(add): preserve user keys & interior comments on tool overwrite

[prostomarkeloff]

TL;DR

write_tool rebuilt [tool.ohbin.tools.<name>] from scratch, so re-publishing an
existing tool silently dropped user-authored keys (e.g. password_committed_ok),
ate interior comments, and could inject a stray blank line before the next section.
It now overwrites an existing entry in place, touching only the generated fields.

The bug

add-gist / add overwrites go through write_tool, which built a brand-new table
from the resolved ToolConfig and kept only the known fields plus the trailing
comment block. Everything else in the table was lost:

• user-authored keys that aren't regenerated — e.g. password_committed_ok
• comments sitting between keys (e.g. a "password intentionally committed" note)
• the blank-line separator before the following section could double up

The fix

When the entry already exists, mutate it in place instead of rebuilding:

• update only the ohbin-owned fields (repo / version / binary / encrypted /
  password) and the per-platform asset hashes;
• leave extra keys, interior comments, and the trailing comment block untouched;
• same platform set (the common re-publish): asset sub-tables are updated
  value-by-value — zero structural change, so surrounding trivia is preserved exactly;
• changed set: reconcile (keep / update / drop / append) rather than
  empty-and-rebuild, which previously perturbed the leading blank line.

Testing

• 2 new regression tests (same-set + changed-set overwrites) assert that the user
  key, the interior comment, the sibling section's layout, and the absence of doubled
  blank lines all survive an overwrite;
• full suite green (70 passed); ruff format + ruff check clean; pyright reports
  0 errors;
• end-to-end: re-running the fixed add-gist against a real consumer's pyproject is
  now idempotent and preserves password_committed_ok.

Bumps the package version to 0.2.3.