chore(credentials): remove legacy plaintext fallback

[cooper (czxtm)]

Summary

Test Plan

Docs

• Docs updated (companion PR in darkmatter/nixmac-web: #___)
• No docs update needed

[cooper (czxtm)]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
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• refactor(frontend): move backend mirrors to viewmodel #255
• chore(beads): update state migration issues #254
• chore(fmt): add treefmt wiring #253
• feat(console): add resizable log panel #252
• fix(theme): scope Minted to Monaco diffs #251
• feat(settings): split backup and tuning panels #250
• chore(credentials): remove legacy plaintext fallback #249 👈 (View in Graphite)
• feat(state): wire runtime slices and summarizer #248
• feat(db): add Diesel pool and table models #247
• feat(state): add scoped slice persistence #246
• refactor(configurable): split derive and generated UI #245
• docs(state): add migration plan issues #244
• implement derive(Configurable) macro #228 : 1 other dependent PR (#229 )
• develop

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[github-actions]

	Warnings
⚠️	Your ## Test Plan section is empty. Describe the steps a reviewer should take to verify this change.

📋 PR Overview

Lines changed	266 (+9 / -257)
Files	0 added, 1 modified, 0 deleted
Draft / WIP	no
Has Test Plan	yes
New UI components	no
New Storybook stories	no
New Rust modules	no
New TS source files	no
New tests	no
package.json touched	no
Cargo.toml touched	no
Infra / CI touched	no

🔬 Coverage

Report	Lines	Statements	Functions	Branches
apps/native/coverage/coverage-summary.json	17.6%	17.6%	31.0%	54.1%

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