docs: consolidate per-tool docs into one root README

[brett-buskirk]

Summary

Second of two PRs for Phase 5 (v0.5.0 — CI & docs, part of #6). Folds docs/*.md into README.md as
one section per tool, per CLAUDE.md's target state ("One clean root README... per-tool docs folded
into a section each").

Most of the original docs/*.md files were one-line stubs (e.g. tcp-client.md: "Basic Python TCP
client") left over from the original raw-repo merge — rather than just copy-pasting those into the
README, I wrote real usage documentation reflecting current behavior: netcat's actual flags/examples
(pulled from its own argparse epilog), tcp-proxy's positional-argv usage, the SSH tools' current
hardcoded defaults and known limitations (ssh-server.py's fixed bind address, the required
test_rsa.key), etc. packet-sniffer's own docs (already thorough) are preserved, trimming only the
now-inapplicable standalone-PyPI-package installation/TestPyPI instructions — superseded by the
toolkit's own single pip install -e ".[dev]" path.

docs/ is removed now that the README is the single source of truth. Added a "Tools" table of
contents at the top linking to each section.

Test plan

• Verified every table-of-contents anchor link against GitHub's actual heading-to-anchor
  conversion rules (lowercase, strip backticks/parens, spaces → hyphens)
• grep'd the whole repo for any remaining reference to the removed docs/*.md paths — none found
• Full pytest suite (179 tests), ruff, and mypy all unaffected and still clean (a docs-only
  change)

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[github-actions]

✅ AgentGate: Passed

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