Add --format md|html|xml to socrates pack

[CryptoJones]

Summary

Adds socrates pack --format {md,html,xml} so the Architect pack can be emitted in formats other than markdown.

Motivation: the planning files (STATE, DECISIONS, RISKS, etc.) must stay markdown — operators edit them in \$EDITOR and they're git-tracked. But the pack is paste-bait for a browser LLM, and there's some evidence that XML-style structural delimiters (or full HTML for table-heavy content) can improve the Architect's reading of the bundle. Putting the option on pack lets users A/B on their own content without disturbing the source-of-truth files.

Formats

Format	Token overhead	Notes
md (default)	baseline	Historical pack behavior; nothing changes for existing users.
xml	~5%	Markdown bodies stay markdown, wrapped in <section> / <header> / <footer> tags with path + label attributes. Matches Anthropic's published recommendation to use XML-style tags for structural delimitation when packing context for Claude.
html	~30-50%	Full HTML, converted from markdown via the optional markdown library. Install via pip install socrates120x[html]. Use when explicitly A/B-testing whether HTML helps your Architect on your content.

Output extension follows the format: .socrates-architect-pack.{md,xml,html}.

Implementation notes

• Refactored pack.py to collect a list of _Section dataclasses in one pass, then dispatch to a format-specific renderer. Same section ordering across all three formats.
• The markdown package is a lazy import (only loaded when the user passes --format html), gated behind a clear RuntimeError that tells the operator how to install it. Added as the [html] optional extra in pyproject.toml.
• XML special chars (<, >, &) in user content are escaped in the XML output so a stray <X> in DECISIONS doesn't break the bundle.
• Fixed a pre-existing minor bug: section labels were leaking the developer's absolute path (e.g. /tmp/pytest-of-akclark/.../AGENTS.md) into the output. Now uses project-relative paths in all formats.

Tests

• 12 new unit tests on build_pack / write_pack: default-is-md, unknown-format-rejection, md headers vs. xml/html tags, XML section attributes, XML escape of user content, HTML doctype/structure, file-extension picking, content survival across all formats, missing-markdown error path.
• 8 new CLI-level tests in a new tests/test_cli_pack.py that drive cli.main directly — the project had no CLI tests before, so this also establishes the harness pattern. Covers: dest/choices/default, stdout-vs-write branching, argparse invalid-choice rejection, the RuntimeError → exit 2 path when markdown is missing, and --help text listing all three choices.

147 tests pass (was 119 before this PR). Ruff clean. Mypy clean.

Out of scope (deliberately)

• No version bump in pyproject.toml. The project's convention is release-summary commits (`v0.7.0: cache segments, preamble template, socrates decide`), not per-feature bumps. Bump to v0.8.0 at release time.
• No CHANGELOG entry — the project doesn't keep one.

Test plan

• pytest -v clean
• ruff check . clean
• mypy src clean
• socrates pack --format xml --stdout from a real project, paste into a Claude/ChatGPT session, confirm Architect can parse the section tags
• pip install socrates120x[html] then socrates pack --format html works
• pip install socrates120x (without [html]), then socrates pack --format html fails with the install hint on stderr and exit code 2

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