docs: Add C4 architecture diagrams (L1–L3) in Mermaid

[shouze]

Context

Parent epic: #24
Depends on: #25 (VitePress + vitepress-plugin-mermaid initialized)

Pages to create in docs/architecture/

overview.md — C4 Level 1: System Context

C4Context
  Person(user, "Developer", "Runs github-code-search from a terminal")
  System(cli, "github-code-search", "Interactive CLI — aggregates GitHub code search results, drives TUI, outputs markdown/JSON")
  System_Ext(github, "GitHub REST API", "Code search endpoint, org team listing")
  Rel(user, cli, "Runs query, navigates TUI, confirms selection")
  Rel(cli, github, "HTTPS — /search/code, /orgs/{org}/teams, /orgs/{org}/teams/{slug}/repos")

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Prose introduction: who uses the tool, what external system it depends on.

containers.md — C4 Level 2: Containers

Show the main processes/modules as containers:

Container	File	Responsibility
CLI parser	github-code-search.ts	Commander subcommands, option parsing, orchestration
API client	src/api.ts	GitHub REST calls, pagination, retry
TUI	src/tui.ts	TTY read/write, keyboard interaction
Output renderer	src/output.ts	Markdown and JSON formatters
Upgrader	src/upgrade.ts	Binary self-replacement

components.md — C4 Level 3: Components (zoom on pure-function core)

Zoom into the pure-function layer called by the CLI and TUI:

Component	File	Key exports
Filter & aggregation	src/aggregate.ts	applyFiltersAndExclusions()
Team grouping	src/group.ts	groupByTeamPrefix(), flattenTeamSections()
Row builder	src/render/rows.ts	buildRows(), rowTerminalLines(), isCursorVisible()
Summary builder	src/render/summary.ts	buildSummary(), buildSummaryFull(), buildSelectionSummary()
Filter stats	src/render/filter.ts	buildFilterStats()
Selection	src/render/selection.ts	applySelectAll(), applySelectNone()
Highlighter	src/render/highlight.ts	highlight()
Output	src/output.ts	buildOutput()

Acceptance criteria

• All 3 pages exist and render without errors (no Mermaid parse errors)
• Each diagram has a prose introduction explaining what it shows
• Component names, arrows and file references match the actual source code
• Diagrams are readable in both light and dark mode