Link W3C Alt Decision Tree URL in alt-text system instruction

[adamsilverstein]

Summary

• Adds the canonical W3C "An alt Decision Tree" URL (https://www.w3.org/WAI/tutorials/images/decision-tree/) inline with the already-named reference in includes/Abilities/Image/alt-text-system-instruction.php.
• Replaces the original parenthetical with an em-dash so we don't nest parentheses around the URL.
• One-line change to the shared system instruction used by every alt-text surface (block editor, media library, bulk action, CLI), which all resolve to the ai/alt-text-generation ability.

Why

Models trained on web content often have strong priors anchored to well-known URLs. Linking the canonical guide gives the model a concrete reference point for the decision framework the instruction explicitly names (decorative / functional / informative / complex). It also documents the authoritative source for anyone reading the prompt later.

What's not in this PR

• No changes to TS callers — they pass only params, not prompt text.
• No changes to Alt_Text_Generation.php or the guidelines injection system — the URL belongs in the instruction itself, not a guideline CPT entry.
• No translation wrapping — the heredoc is intentionally untranslated (system instruction for the model, not user-visible UI).

Closes #461

Test plan

• php -l includes/Abilities/Image/alt-text-system-instruction.php passes
• Trigger alt-text generation via block editor on a local WP install with an AI provider configured; output still sensible
• Trigger alt-text generation via media library modal; output still sensible
• E2E specs run clean:
  • tests/e2e/specs/experiments/alt-text-generation.spec.js
  • tests/e2e/specs/experiments/alt-text-generation-bulk.spec.js

[github-actions]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.72%. Comparing base (4c9699f) to head (dbd92e4).

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #462   +/-   ##
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  Coverage      67.72%   67.72%           
  Complexity       959      959           
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  Files             60       60           
  Lines           4836     4836           
==========================================
  Hits            3275     3275           
  Misses          1561     1561           

Flag	Coverage Δ
unit	67.72% <ø> (ø)

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[adamsilverstein]

Closing — the inline decision tree in the system instruction already specifies the decorative/functional/informative/complex rules explicitly, so adding the URL is unlikely to change model output. Not worth the churn for documentation-only value. Closing alongside #461.