BIP 89: Chain Code Delegation for Private Collaborative Custody (#2004)

* Add Chaincode Delegation BIP

* Update license to BSD-3-Clause and expand blinded signing documentation

* Address initial PR comments

* Update with BIP number assignment

* Fix delegator_sign test vector

* Upgrade secp256k1lab and add license file

- Upgrade vendored secp256k1lab to commit a265da1 (adds type annotations)
- Add COPYING file to satisfy MIT license requirements
- Document secp256k1lab commit reference in BIP text

* Fix type checker and linter issues in reference implementation

- Fix TweakContext to use Scalar types for gacc/tacc
- Replace HashFunction enum with Callable type alias
- Fix bytearray to bytes conversion in blind_sign
- Move imports to top of file
- Fix boolean comparison style (use 'not' instead of '== False')
- Add proper type annotations and casts for dict handling
- Remove unused imports and type ignore comments

* Address PR review comments on terminology and clarity

- Add intro explaining delegation naming (chain code is delegated, not
  signing authority)
- Reorder terminology to list Delegator before Delegatee
- Replace "quorum" with clearer "can co-sign for UTXOs" language
- Clarify derivation constraints in terms of delegatee's extended key
- Rename "Delegatee Signing" section to "Signing Modes"
- Fix "delegatee can apply" to "delegator can produce" (line 112)
- Replace undefined "caller" with "delegatee" (line 173)
- Clarify "Change outputs" to "Tweaks for change outputs" (line 98)
- Add note that message is separate from CCD bundle
- Add note on application-specific verification (addresses, amounts)
- Add transition sentence clarifying non-concurrent protocol scope

* Add changelog entry for 0.1.3

* Fix header: use Authors (plural) for multiple authors

* Fix BIP header format for CI compliance

- Change Type from 'Standards Track' to 'Specification' (valid type)
- Change 'Created' to 'Assigned' (correct field name per BIP format)
- Change 'Post-History' to 'Discussion' (recognized field in buildtable.pl)

* Apply suggestion from @murchandamus

---------

Co-authored-by: Jesse Posner <jesse.posner@gmail.com>

README.mediawiki

@@ -512,6 +512,13 @@ users (see also: [https://en.bitcoin.it/wiki/Economic_majority economic majority
 | Dmitry Petukhov
 | Informational
 | Complete
+|-
+| [[bip-0089.mediawiki|89]]
+| Applications
+| Chain Code Delegation
+| Jesse Posner, Jurvis Tan
+| Specification
+| Draft
 |- style="background-color: #cfffcf"
 | [[bip-0090.mediawiki|90]]
 |