fix: actionable errors for malformed --data instead of a raw traceback

[igor-ctrl]

Summary

• post, patch, and action each carried their own copy of an unguarded json.loads() for --data/-d. A shell-mangled inline literal (PowerShell strips quotes from {"a": 1} routinely) or a bare file path passed without the @ prefix surfaced a ~25-line raw json.JSONDecodeError traceback instead of telling the user what to fix.
• Extracted the shared parsing logic into bcli_cli._data_arg.parse_data_argument() (the codebase already had a pattern of single-purpose _*.py helper modules shared across commands — _safety.py, _url_resolve.py, _envelope_wrap.py — so this follows suit rather than leaving three drifting copies).
• Every failure now raises typer.BadParameter (never a bare JSONDecodeError): the message includes the JSON error's line/column/reason and a short excerpt of what was received (truncated to ~80 chars, never the whole payload), plus a one-line hint when the input looks like a filesystem path (missing @) or a shell-mangled literal (starts with { but has no quotes, or has unquoted keys).
• A malformed @file now names the file in its error. A missing @file keeps the existing "File not found" message unchanged.

Changes

• src/bcli_cli/_data_arg.py — new shared helper, parse_data_argument().
• src/bcli_cli/commands/{post,patch,action}_cmd.py — use the shared helper, removed the three duplicated _parse_data() definitions.
• tests/test_cli/test_data_arg.py — new, full heuristic matrix for the shared helper.
• tests/test_cli/test_post_cmd.py, tests/test_cli/test_patch_cmd.py — new, command-level coverage (these two commands had no dedicated test file before).
• tests/test_cli/test_action_cmd.py — extended TestDataHandling with the same error-path cases.
• CHANGELOG.md — [0.8.1] entry.
• pyproject.toml / uv.lock — version bump to 0.8.1 (src/bcli/_version.py reads from package metadata at runtime, so it needed no change).
• docs/command-reference.md, docs/write-operations.md — documented that @ is required (a bare path is parsed as JSON, not silently treated as a file) and the PowerShell quote-stripping gotcha.

Test plan

• uv run --extra dev python -m pytest tests/ -q — 1131 passed, 5 skipped (pre-existing, unrelated), 3 warnings (pre-existing, unrelated to this change).
• uv run ruff check src/ tests/ — all checks passed.
• Manually exercised all target scenarios against parse_data_argument() directly (valid inline, valid @file, shell-mangled with/without quotes, bare existing file path, Windows drive path, path-like-but-missing, missing @file, malformed @file, generic garbage) — every failure path raises typer.BadParameter with the expected hint, none leak a raw JSONDecodeError.