fix(fugu): accept a bare integer access index in workflow parse-gate

[rsnetworkinginc]

Summary

The Fugu workflow parse-gate normalizes each step's access entry through
_normalize_access (src/trinity/fugu/workflow.py). It accepted the string
form ("0"), the list form ([0]), and the "all"/empty shorthands, but a
bare integer access index (0) fell through every branch and returned
None — which rejects the entire workflow as a parse failure.

A bare int is the natural shorthand for a single prior-step index, so a
Conductor emitting a perfectly valid proposal like:

model_id    = [0, 1, 2]
subtasks    = ["solve", "check", "answer"]
access_list = ["none", 0, 1]

was scored r = 0 even though "none", "0", and [0] in the same position
all parse fine. This penalizes correct workflows for a cosmetic formatting
choice the parser never documented as invalid.

Closes #225.

Fix

Handle a bare int in _normalize_access as a single index, mirroring the
existing string-digit branch:

• bool is rejected first (it is an int subclass and never a valid index).
• an int j is accepted as [j] only when 0 <= j < step_index, so
  forward references and negative indices still reject the workflow (same DAG
  invariant as the list branch).

The docstring is updated to list the bare-integer form as valid.

Tests

Added test_parse_accepts_bare_int_access_index (RED before the fix, GREEN
after):

• access_list = ["none", 0, 1] now parses to [[], [0], [1]].
• a bare-int forward reference (access_list = [0, 1]) is still rejected.

$ python -m pytest tests/test_fugu_workflow.py -q
.........                                                                 [100%]
9 passed

No behavior change for any previously-accepted or previously-rejected form.