feat(local-feature-flag-evaluator): document starts_with/ends_with local-eval operators

[posthog]

Why

Six SDKs independently shipped local-evaluation support for the same new property-filter
operators within the same week, all citing the same server-side change:

• posthog-js#4342 (@posthog/core, shared by posthog-node/posthog-js-lite)
• posthog-python#820
• posthog-php#209
• posthog-ruby#229
• posthog-go#264
• posthog-dotnet#268
• Server-side: PostHog/posthog#72992

Before these PRs, flags using starts_with / not_starts_with / ends_with / not_ends_with
property filters could not be evaluated locally in any of these SDKs — every access fell back to
remote evaluation. All six implementations converge on the same semantics: stringify both sides,
ASCII-lowercase, then compare with a prefix/suffix check (negated for the not_* variants) —
mirroring the SDKs' existing icontains handling.

openspec/specs/local-feature-flag-evaluator/spec.md describes property-filter matching only at
the behavioral level and never enumerated which operators a conformant local evaluator must
support (not even icontains). This is genuinely missing coverage, not a contradiction of
existing text.

What this PR does

• Adds one new requirement, String prefix/suffix property filter operators, with two
  scenarios (a matching case and a missing-property inconclusive case), following this repo's
  propose → apply → archive convention in one PR.
• Adds a one-line pointer from Behavior item 6 to the new requirement.
• openspec validate --specs --strict passes (59/59).

Uncertain / flagged for reviewer attention

• Only the 6 SDKs above were checked. posthog-android, posthog-ios, posthog-java
  (posthog-server), and posthog-flutter were not audited for the same operator support in this
  pass — noted as a follow-up in tasks.md §4.1.
• posthog-dotnet's PR also changes how an unrecognized operator is handled (degrades that one
  flag to remote evaluation instead of disabling local evaluation project-wide). Only one SDK
  was confirmed to implement this resilience behavior, so it's intentionally left out of this
  change's scope (see tasks.md §4.2) rather than generalized from a single data point.

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