feat(claude): add Claude Fable 5.1 model

[rynfar]

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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[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.6 KiB	13.6 KiB	+10 B (+0.1%)	15.1 KiB	✅
Codex	Thread snapshot wire	6.9 KiB	6.9 KiB	+10 B (+0.1%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.6 KiB	6.6 KiB	0 B (0.0%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	57.2 KiB	57.2 KiB	0 B (0.0%)	66.4 KiB	✅
Codex	Live turn messages	10	10	0 (0.0%)	21	✅
Claude	Total thread wire	13.6 KiB	13.6 KiB	−18 B (−0.1%)	15.1 KiB	✅
Claude	Thread snapshot wire	6.9 KiB	6.9 KiB	+3 B (+0.0%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.6 KiB	6.6 KiB	−21 B (−0.3%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	58.1 KiB	58.1 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	10	10	0 (0.0%)	21	✅

Baseline: a509639 · PR result: fe0fe86 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 109.5 KiB
• Claude decoded thread snapshot: 110.2 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.