fix(strands-command): inline full PR diffs within a total budget

[yonib05]

Problem

get_pr_files capped every changed file's patch at 50 lines:

if len(patch_lines) > 50:
    output += '\n'.join(patch_lines[:50])
    output += f"\n   ... (truncated, {len(patch_lines) - 50} more lines)\n"

The reviewer agent then reasons about a partial diff for any non-trivial
change. A bug introduced on line 60 of a changed file is simply invisible
to it, and the agent gets no signal that anything is missing beyond a
terse "truncated" note. This silently caps review quality on exactly the
larger PRs that most need a careful read.

Fix

Replace the blunt per-file line cap with a total diff character budget:

• inline each file's full patch until the cumulative budget is reached,
• once exceeded, list the remaining files (with their add/delete stats)
  and instruct the agent to fetch them on demand via the shell tool,
  rather than cutting a file off mid-diff.

This keeps the original intent (bounding how much diff goes into one
prompt) while ensuring that whatever is inlined is complete, and that
anything omitted is explicitly surfaced instead of hidden.

Tests

Adds tests/test_github_tools.py:

• full diff (120 lines) is inlined without truncation,
• oversized total diff lists the file for on-demand fetch,
• binary/no-patch files are handled.

The first test fails against the old code (it emitted truncated and
dropped the tail), confirming the regression it guards.

35 passed