feat(claude): add Claude Fable 5.1 model - #247

Pylon's Claude catalog stops at Fable 5, so Claude Fable 5.1 cannot be selected
even on a Claude Code new enough to run it. Upstream added the model in
pingdotgg/t3code#9078; this
adopts that change and nothing else.

claude-fable-5-1 joins the Claude model catalog with the effort and
context-window descriptors, gated behind Claude Code v2.1.257 so older CLIs
hide the model rather than failing at spawn time. The version-upgrade message
chain now leads with Fable 5.1, xhigh effort is preserved for the model
instead of being widened to max, and fable / fable-5.1 /
claude-fable-5.1 resolve to the canonical slug.

Scope

Cherry-picked with -x from c17d02cff, clean, no conflicts, applied
line-for-line. It is self-contained and does not depend on
#9084 (remote Claude model
manifest discovery), the much larger ClaudeProvider.ts restructure that
follows it upstream. That work is deliberately out of scope here.

This is a scoped adoption, not a batch review: 128 other upstream commits in
9b2d0431..57a66608 remain unreviewed, so .agents/upstream-review.md records
the decision without advancing reviewed-through.

Worth a reviewer's attention

The bundled manifest swap puts claude-fable-5-1 into the current Claude set in
place of claude-fable-5, so Fable 5 now reports as legacy. That is
upstream's intent — 5.1 supersedes 5 in the same tier at the same per-token
price — but it is a visible status change to an existing model.

Verification

• vp test run over the four touched test files: 4 files, 170 tests, all
  passed, none skipped.
• vp run -F t3 -F @t3tools/contracts -F @t3tools/shared typecheck: exit 0,
  3/3 packages checked, no errors and no new diagnostics in the touched files.
• Checked that nothing else in Pylon hardcodes Claude model slugs; usage pricing
  keys on the bare fable family, so it prices 5.1 with no change.
• Not done: a real-client pass on the model picker. Happy to run one on request.

Written by Claude Opus 5 in Claude Code.

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