docs(adr): mark ADR-0002 and ADR-0003 as amended by ADR-0005

[fercodes]

Closes #75

Summary

Adds amendment notes to docs/adr/0002-agent-registry-on-soroban.md and docs/adr/0003-payment-rail-and-custody.md so every row of ADR-0005's "What this supersedes or amends" table maps to a note at the affected section, naming the decision ID (D1/D2/D4/D5) and whether the section is superseded or amended. Original decision text is unchanged; the diff is additions-only.

Acceptance criteria

• Both headers say they are amended by ADR-0005, with a link
• Every ADR-0002/ADR-0003 row of the ADR-0005 amendments table has a matching note at the right section
• Each note says "superseded" or "amended" and names the ADR-0005 decision ID
• The original decision text is not rewritten or deleted
• All links resolve on GitHub

Verification evidence

$ git diff --numstat
13	0	docs/adr/0002-agent-registry-on-soroban.md
21	0	docs/adr/0003-payment-rail-and-custody.md

$ git diff --unified=0 | grep '^-' | grep -v '^---' || echo "None (fully additive)"
None (fully additive)

Notes added (decision ID / status):

• ADR-0002: registries alignment (D5, amended); payment-backed feedback (D5, superseded); Reputation Registry section + authorize_feedback (D5, superseded)
• ADR-0003: decision 1 (D1, D2, superseded); decision 2 (D1, amended); decision 3 (D1, superseded); decision 6 (D1, D4, amended); #55 consequence row (D1, superseded); alternative 3 (D1, superseded); decisions 4–5 (unchanged)
• Anchor checked: docs/adr/0005-align-agent-commerce-with-erc-8183-and-erc-8004.md → #what-this-supersedes-or-amends

Notes for reviewers

Docs-only change, so no component CI jobs apply. Out of scope per #75: rewriting the ADRs, the #74 Identity Registry gap review, the #14 redesign, and ADR-0001/vision updates.

[TOMOKI977]

Approved. Every ADR-0002/ADR-0003 row of the ADR-0005 amendments table has a matching note at the right section, and the diff is purely additive.

One small nit: the doc comment on authorize_feedback says "Amended by ADR-0005 D5", but the ADR-0005 table marks that part as Superseded. Worth aligning the wording so the two don't disagree.

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[TOMOKI977]

@fercodes merged into main as a3cc577 (squash), which closes #75. Thanks for keeping the notes purely additive, it made the review easy.

I had to update the branch from main twice before merging (it fell behind after #99 and #100 landed), so your local docs/75-adr-amendments is out of date. Feel free to delete it, and the remote branch too if you don't need it.