fix(miner-extension): purge a stale discoveryIndexUrl value from chrome.storage.sync

[JSONbored]

Summary

#5343 removed the discoveryIndexUrl UI field and stopped reading/writing it, but chrome.storage.sync.set only merges keys — it never deletes ones an earlier extension version already synced. Without an active purge, a value synced before #5343 stays in a user's account indefinitely, which is a real privacy/data-retention gap #5343 didn't close.

• Add removeLegacyDiscoveryIndexUrl() to apps/gittensory-miner-extension/options.js, called from refreshSettings() (which runs on every options-page load and again at the end of every save), so any stale value is cleared regardless of which path a user hits first.
• Extend the existing dead-field regression test in test/unit/miner-extension-content.test.ts with coverage for the purge on both load and save, plus a direct check that the internal helper is exposed for testing.
• Also regenerates packages/gittensory-miner/docs/env-reference.md, which was already stale on main (GITTENSORY_MINER_KILL_SWITCH from Document and test .gittensory-miner.yml config precedence against MINER_* env vars and CLI flags #5198/docs(miner): document and test .gittensory-miner.yml config precedence (#5198) #5500 was never regenerated in) — unrelated to this fix but required for miner:env-reference:check to pass in this PR's own CI. Also fixed standalone in docs(miner): regenerate env-reference.md for GITTENSORY_MINER_KILL_SWITCH #5510, so this hunk will disappear on rebase once that merges.

Scope

• In scope (apps/gittensory-miner-extension/, test/, plus the required generated-doc regeneration), narrow
• No secrets/private terms
• General fix (purges the key from every load/save path), not a special-cased one-off

Validation

• npx vitest run test/unit/miner-extension-content.test.ts — 12/12 passing
• npm run build:miner — clean
• npm run test:ci — full local gate green (795 test files / 15432 tests passed, 12 skipped)
• npm audit --audit-level=moderate — 0 vulnerabilities
• Confirmed via git show origin/main:...options.js that the gap is still open on current main (no purge call anywhere)

Safety

• No secrets in code/comments/tests
• chrome.storage.sync.remove on an already-absent key is a documented no-op — safe to call unconditionally on every load/save

[superagent-security]

Superagent did not find any vulnerabilities or security issues in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.75%. Comparing base (97476d9) to head (d174f3f).
⚠️ Report is 7 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5511   +/-   ##
=======================================
  Coverage   94.75%   94.75%           
=======================================
  Files         565      565           
  Lines       44914    44914           
  Branches    14675    14675           
=======================================
  Hits        42559    42559           
  Misses       1621     1621           
  Partials      734      734           

Flag	Coverage Δ
shard-1	43.78% <ø> (-0.51%)	⬇️
shard-2	35.40% <ø> (+0.05%)	⬆️
shard-3	32.09% <ø> (+0.15%)	⬆️
shard-4	31.37% <ø> (-0.58%)	⬇️
shard-5	33.32% <ø> (+0.38%)	⬆️
shard-6	43.81% <ø> (+0.27%)	⬆️

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