feat(tui): show compaction summary in context dialog

[Sayt-0]

Summary

• expose the latest compaction summary through the context breakdown
• display the full summary in the scrollable /context dialog
• include the verbatim summary in /context clipboard output
• derive the displayed summary and prompt messages from the same session-history snapshot

Closes #3870.

Preview

Approximate plain-text rendering of /context after a compaction. The actual TUI applies the existing category colors, border, usage bar, and scrollbar styles.

╭────────────────────────────────────────────────────────────────────╮
│ Context Window                                                     │
│ openai/gpt-4o  •  limit: 128.0K tokens                             │
├────────────────────────────────────────────────────────────────────┤
│ █████████████░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░ │
│ ~25.4K of 128.0K tokens (20%)                                      │
│                                                                    │
│ ■ System prompt          1.2K    <1%  (3 messages)                 │
│ ■ Tool definitions       8.4K     7%  (23 tools)                   │
│ ■ Prompt files            600    <1%  (1 file)                     │
│ ■ Messages               6.1K     5%  (12 messages)                │
│ ■ Tool results           8.2K     6%  (18 results)                 │
│ ■ Compaction summary      900    <1%  (1 summary)                  │
│ ■ Free space           102.6K    80%                               │
│                                                                    │
│ Latest compaction summary                                          │
│ The user is implementing issue #3870. The chosen UX is to expose   │
│ the latest compaction summary from the /context dialog. The        │
│ implementation must preserve scrolling and clipboard support.      │
│ Tests and linting currently pass.                                  │
│                                                                    │
│ Token counts are estimates; the provider's tokenizer may count     │
│ differently.                                                       │
│                                                                    │
│ ↑↓ scroll                         c copy                  Esc close │
╰────────────────────────────────────────────────────────────────────╯

When live sessions or file inventories exist, the summary is placed after those selectable sections so their navigation mapping remains unchanged:

Live sessions
▶ root       a81c04f2  25.4K of 128.0K (20%)  (current)
  developer  531cb912  12.1K of 128.0K (9%)

Attached files
  main.go     2.1K   2%  /project/main.go

Latest compaction summary
The latest summary wraps to the dialog width and remains scrollable.

The section is omitted when the session has never been compacted. Pressing c copies the same summary verbatim as plain text.

Issue expectations

Expectation	Implementation
View a compaction result summary	/context renders the latest stored compaction summary
Understand what the model retained	The exact summary injected into subsequent model context is shown verbatim
Preserve usability for long summaries	Text wraps to the dialog width and remains inside the existing scroll view
Work after session persistence/resume	The view reads the already persisted session summary

A semantic retained-versus-dropped diff and agent-level persistent compaction instructions are intentionally excluded. FirstKeptEntry identifies the verbatim retained message boundary, but it cannot prove which facts a summarizing model preserved semantically.

Validation

• task test
• go test -race ./pkg/session ./pkg/runtime ./pkg/tui/dialog
• ./scripts/build.sh
• golangci-lint run
• go run ./lint .
• go mod tidy --diff
• gofmt -l .