Add error code reference and cross-link troubleshooting guide

[Mathew2k-hash]

Add normative error-code reference and cross-link troubleshooting guide

Fixes the "support keeps re-typing SDK errors in Discord" problem by giving every error a
stable, linkable anchor.

What's new

reference/error-codes.mdx — a new normative reference page with one table per error source:

• Soroban contract errors (HostError: Error(Contract, #N)) for stealth-registry,
  stealth-sender, and wraith-names — symbolic name, meaning, common cause, and fix for every
  variant
• SDK Agent Client errors — err.message patterns thrown by Wraith / WraithAgent
• SDK Stellar Chain Primitives errors — throw-sites in @wraith-protocol/sdk/chains/stellar
  (meta-address decoding, key derivation, ECDH edge cases)
• CLI errors — all five WRAITH_* exit codes
• Soroban RPC / indexer errors — retention_window_exceeded, start_ledger_too_old,
  invalid_cursor, contract_not_found, ledger_not_found
• Quick-reference cross-index mapping all 20 stellar-troubleshooting.mdx entries to the
  canonical row in this table

Cross-links added to guides/stellar-troubleshooting.mdx

Top-of-file callout pointing to the new reference, plus inline Reference: lines at sections 7,
13–20 linking directly to the matching table anchor.

Nav

reference/error-codes added as the first entry in the Reference group in docs.json.

Auto-generation note

The page documents a generate:error-codes script pattern (mirroring
scripts/generate-stellar-reference.ts) for keeping contract error tables in sync from
TypeScript bindings once the script is implemented.

Testing

pnpm run check:snippets passes — 60 MDX files, 572 snippets, 0 failures. The new page has no
code fences so it adds no new CI surface.
closes #121

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[truthixify]

Thanks @Mathew2k-hash. #135 (nav cleanup) merged and restructured docs.json, so that is now your only conflict. reference/error-codes.mdx and the troubleshooting cross-links are clean.

git fetch origin
git rebase origin/develop

On the conflict, take the new nav tree from develop and re-add your Reference entry into it rather than keeping your old version of the file.

[truthixify]

I have rebased this onto develop and force-pushed to your branch, since the conflict was purely mechanical and you should not have to burn a round trip on it. Your commits are unchanged, only replayed.

The only conflict was the Reference group in docs.json. develop had grown to six pages while this was open, so I took that list and inserted reference/error-codes into it rather than taking either side wholesale.

Verified locally before pushing: check:nav-coverage passes (68 pages, 68 nav entries) and check:snippets passes (600 snippets).

Nothing needed from you unless you disagree with where the nav entry landed. If you have a local copy of this branch, git fetch && git reset --hard origin/feat/error-codes-reference before you continue.

[truthixify]

Merged. Thanks for your patience on the rebase @Mathew2k-hash. Pairing the normative table with cross-links back into the troubleshooting guide is what makes both pages more useful rather than redundant.