arch: turn ita into a proper Python package (#256)

[voidfreud]

Summary

Promotes ita from a sibling-of-_*.py-modules script into a proper Python package. Drops the sys.path.insert hack. Standard packaging end-to-end.

Closes #256. Part of meta-issue #334. Refs CONTRACT §15.

Layout change

src/ita.py        (deleted — was the inline-script launcher)
src/__init__.py   → src/ita/__init__.py    (exposes `cli`, imports all command modules)
src/__main__.py   → src/ita/__main__.py    (enables `python -m ita`)
src/_*.py         → src/ita/_*.py          (21 modules)

What changes

• Internal imports: from _foo import X → from ._foo import X everywhere (top-level + in-function).
• Test imports of internals: from _core import X → from ita._core import X in 4 test files; in-function imports likewise.
• Subprocess invocations (~10 test files hardcoded src/ita.py): switched to python -m ita. Canonical via tests/helpers.py (ITA_CMD list); a back-compat ITA = ['python', '-m', 'ita'] list stays for tests that splat their own argv.
• pyproject.toml: adds [build-system] (hatchling) and [tool.hatch.build.targets.wheel] packages = ["src/ita"]. [project.scripts] ita = "ita:cli" now resolves to the package. mutmut path updated.
• install.sh: rewritten as uv tool install --force --from . ita. uv manages the launcher in ~/.local/bin and rebuilds when re-run. No more symlinks.
• README: install + dev sections updated.
• .gitignore: add dist/ and build/.

Verification

• uv build --wheel → dist/ita-0.7.0-py3-none-any.whl ✓
• python -m ita --version → 0.7.0 ✓
• Fast-lane pytest (-m "not integration and not stress and not perf and not fault_injection"):
  • Branch: 14 fail / 118 pass
  • main baseline: 14 fail / 118 pass
  • Identical failure list — all pre-existing @regression markers for open issues Phase 3 will fix (bug: broadcast add replaces all existing domains instead of merging #220 bug: on prompt timeout silently exits 0 with no output #247 bug: broadcast on reports success but broadcast list shows empty #249 bug: ita new returns before shell metadata is stable #250 CRITICAL: bulk clear --where/--all bypasses protection check #283 bug: broadcast send can duplicate delivery when session is in multiple domains #285 proposal: formal prompt-detection contract #288 bug: timeout exit code is mode-dependent — JSON says 124, process exits 1 #290 + broadcast suite + test_cwd_then_run_order).
  • Zero new regressions.

Behavioural change for users

• The ita command is now installed via uv tool install, not symlinked. After re-pulling, you must re-run ./install.sh to update the launcher (uv tool installs are wheel snapshots, not editable). For dev work, use uv run python -m ita ... directly from the repo.

Test plan

• Wheel builds clean
• python -m ita --version works
• Fast-lane test delta vs main: 0
• Post-merge: ./install.sh end-to-end on a clean clone (user-side)
• Post-merge: SKILL.md update is now safe to plan (deferred per project rule until everything else lands)

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