docs: update codec ownership guidance from frozen spanenc to spancodec #289

Problem

The package ownership guidance still directs new reflection/client-tag encoding work to spanenc, which is now frozen and superseded by spancodec.

Verified at spanvalue main 788c5425113def54d55e7c951c0a371bfa5c3418:

• gcvctor godoc recommends github.com/apstndb/spanenc for arbitrary Go values and client encoding semantics.
• AGENTS.md adoption boundaries names spanenc as the owning module.

However, spanenc's module deprecation notice says v0.3.x is frozen and development moved to spancodec. The spancodec migration guide documents its replacement of both spanenc and spandec, including the option-signature differences.

This can route new consumers and follow-up fixes to the retired module. The historical spanenc introduction in #233 was correct at that time; this issue tracks the subsequent ownership change.

Proposed scope

• Point current gcvctor package godoc and agent adoption boundaries to spancodec.
• Keep gcvctor's explicit constructor responsibility and structfields' separate upstream-derived code/license boundary intact.
• Link the migration guide; do not imply a universal import-only migration because option-slice signatures differ.
• Do not introduce a codec dependency or move codec functionality into spanvalue merely to update the guidance.

Validation: verify rendered godoc links and run the documentation/lint gate. Evidence grade: source-backed documentation drift; no runtime behavior change is proposed.