chore(config): T-1258 — tooling/ becomes an importable package

The skeleton the reach CLI hangs off. Nothing moves yet: this adds the
package, the bounded core/, and explicit setuptools discovery.

core/console.py is the single output path, and the split it enforces is the
whole design — stdout carries the command's actual output so `reach ... | jq`
keeps working, stderr carries the event stream as JSONL. Rendering happens at
the sink: a terminal gets human text, anything else gets raw JSONL, so a live
view and a job log are one artefact in two presentations. Emitting is
optional — the gates emit nothing — and verdict() prints once, last, carrying
its remedy as a structured field.

core/config.py resolves the repo root from __file__ against a project.yaml
sentinel, with an SR_REPO_ROOT override. No subprocess and no git call: this
is on the gate path, and cwd is not a reliable signal anyway since a hook runs
from the root and an agent call may not. Both paths are validated, because a
silent fallback is how you end up editing one checkout and checking another.

Discovery is configured explicitly rather than left to flat-layout
auto-discovery, which would have had to choose between erroring on the
ambiguity and quietly shipping client/ or docs/. Verified: top_level.txt
contains exactly "tooling".

Verified beyond the happy path — the sentinel rejects SR_REPO_ROOT=/tmp and
names both remedies; debug events are suppressed at the default threshold
while the verdict is not; stdout stays clean with stderr redirected away; and
the three unconditional push-gate checks still pass now that tooling/ is a
package, which was the real regression risk.

Two findings recorded on the tickets. make setup-venv is stale — it calls
.venv/bin/pip, but the venv was created by uv and has no pip, so the recorded
procedure and the actual state have already diverged (T-1261 owns the fix).
And settled-reach-tooling had never actually been installed: site-packages
held the dependencies but no dist-info, which follows from there being no
__init__.py to expose. This is the first commit where `import tooling` means
anything.

.venv/ was only ignored via .git/info/exclude, which is machine-local, so a
fresh clone or a new worktree did not ignore it at all.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tooling/core/config.py

@@ -0,0 +1,63 @@
+"""Repo-root and path resolution. No domain logic lives here (D-263).
+
+This module is on the push-gate path, so it does no subprocess work: no
+`git rev-parse`, no shelling out, nothing beyond the stdlib. Resolution is
+`__file__`-relative and validated against a sentinel file. That is not only
+faster than asking git — it is *more correct*, because the current working
+directory is not a reliable signal. A git hook runs from the repo root, an agent
+`Bash` call may not, and `reach` is meant to work from anywhere.
+"""
+
+from __future__ import annotations
+
+import os
+from pathlib import Path
+
+# The file whose presence proves a directory is the repo root. project.yaml is
+# the version source of truth (CLAUDE.md), so it is the honest sentinel: if it
+# is absent, everything downstream was going to fail anyway — better to say so
+# here, by name, than to hand out a plausible wrong path.
+SENTINEL = "project.yaml"
+
+ENV_OVERRIDE = "SR_REPO_ROOT"
+
+
+def repo_root() -> Path:
+    """Absolute path to the repo root.
+
+    `SR_REPO_ROOT` wins if set; otherwise the package's own location is used.
+    Both are validated, deliberately: an override pointing somewhere useless
+    should fail loudly rather than fall back to a default that happens to work,
+    because a silent fallback is how you end up editing one checkout and
+    checking another.
+
+    Not cached. The two syscalls are microseconds, and a cache would make the
+    override untestable for the sake of nothing measurable.
+    """
+    override = os.environ.get(ENV_OVERRIDE)
+    if override:
+        return _validated(Path(override).expanduser().resolve(), f"{ENV_OVERRIDE}={override}")
+    # tooling/core/config.py -> tooling/core -> tooling -> repo root
+    return _validated(Path(__file__).resolve().parents[2], "the installed package location")
+
+
+def path(*parts: str) -> Path:
+    """Join `parts` onto the repo root, so callers do not each re-resolve it."""
+    return repo_root().joinpath(*parts)
+
+
+def _validated(root: Path, source: str) -> Path:
+    if (root / SENTINEL).is_file():
+        return root
+    # Raised as RuntimeError only because core/errors.py does not exist yet;
+    # T-1249 converts this to ReachError(message, fix=...). The message already
+    # follows the contract — it names the command that fixes it.
+    raise RuntimeError(
+        f"cannot locate the repo root: {root} contains no {SENTINEL} "
+        f"(resolved from {source}).\n"
+        f"If reach was installed from a different checkout than the one you are "
+        f"working in, re-point it:\n"
+        f"    uv tool install --editable <path-to-repo>\n"
+        f"To override for a single command:\n"
+        f"    {ENV_OVERRIDE}=<path-to-repo> reach ..."
+    )