feat(scripts): support bracket-quoted keys in the example-validator JSONPath subset

[douglasborthwick-crypto]

Problem

extract= and target= navigate the displayed example with a small JSONPath subset. That subset splits the path on . and matches each segment against ^(\w+)(?:\[(\d+)\])?$, so a segment can only ever be a bare identifier.

UCP uses reverse-domain identifiers and scope tokens as object keys. Their dots are read as path separators and their colons match no segment, so those keys cannot be addressed at all:

<!-- ucp:example schema=common/identity_linking extract=$.config.providers.com.example.attestor[0] -->

ERR  repro.md:17  [schema=common/identity_linking extract=$.config.providers.com.example.attestor[0] op=read]
     extract path not found: $.config.providers.com.example.attestor[0]: 'com'

The same applies to $.capabilities.dev.ucp.common.identity_linking[0] (KeyError: 'dev') and to scope tokens such as dev.ucp.shopping.order:read. There is no escaping form today — ['...'] is not recognised either, so an example whose payload sits under a capability name, a provider key, or a scope token has no usable extract=/target= expression.

Change

Adds the standard bracket-quoted form alongside the existing spellings:

extract=$.config.providers['com.example.attestor'][0]
target=$.capabilities['dev.ucp.common.identity_linking'][0]

Single and double quotes are both accepted.

Path parsing moves into one split_path() helper shared by jsonpath_get, jsonpath_set, jsonpath_get_schema and jsonpath_to_pointer, so all four agree on the grammar. Previously each re-implemented the split inline. Unparsable segments still raise rather than silently mis-navigating.

Compatibility

Bare and indexed paths are unchanged. The doc corpus validates identically before and after — 343 passed, 0 failed, 0 errors, 50 skipped on both main and this branch.

Tests

Adds 10 unit tests to scripts/test_validate_examples.py (52 → 62 passing) covering:

• bare / indexed / $ regression
• dotted quoted keys and reverse-domain capability names
• colon-bearing scope tokens
• both quote styles
• jsonpath_set writing through a quoted key
• JSON Pointer conversion of a quoted key (elision-path reporting)
• the unquoted-dotted-key error path still raising

Docs

Documents the subset in the validator's contract docstring and in docs/documentation/schema-authoring.md.

Verification

Run locally against ucp-schema 1.4.1:

	main	this branch
ucp-schema lint source/	✓ 124 files	✓ 124 files
validate_examples.py	343 / 0 / 0 / 50	343 / 0 / 0 / 50
test_validate_examples.py	52 passed	62 passed
ruff check + ruff format --check	clean	clean