docs: identify and catalogue tech debt as Epic 7

[Copilot]

Systematic audit of the codebase to identify and document accumulated technical debt, structured as a new epic matching the existing issues format (Epics 1–6).

What was found

Severity	Issues
🔴 Critical	Monolithic useChordState (309 lines, 7 states, mixed concerns); shared/ChromaticCircle.tsx unused duplicate; scale computation logic duplicated across two feature modules; hand-written API wrappers co-mingled with auto-generated types
🟠 High	Chord-note retrieval pattern copy-pasted in 5+ files; zero direct tests for any hook; no React Error Boundary; magic numbers (1200, 1500, sRGB gamma constants, inline hex colors); nullable context defaults forcing unsafe casts in every consumer
🟡 Medium	Inconsistent folder structure across 15 feature modules; no test:coverage / test:watch scripts; no measurable coverage baseline

Deliverables

issues/epic-7-tech-debt/epic7.md — Epic overview with baseline state table, full issue index, dependency graph, and architecture notes for the three most complex items.

12 individual issue specs (ISSUE-E7-01 → ISSUE-E7-12), each with: objective, background, concrete file list, and acceptance criteria. Issues are sized XS–L and ordered with an explicit dependency graph:

E7-04, E7-05, E7-06, E7-07, E7-10, E7-12  ← fully independent, parallelisable

E7-01 ──┬──► E7-08
         └──► E7-11
E7-02 ──┬──► E7-09
         └──► E7-11
E7-03 ──────► E7-08

No source code was changed.

Original prompt

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This section details on the original issue you should resolve

<issue_title>identify tech debt</issue_title>
<issue_description>Look for areas of improvement in the codebase. Document your findings for a future epic. Do not make code revisions. </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes identify tech debt #251

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