feat(tui): add `/context` command with a categorized context-window breakdown

[aheritier]

Summary

Add a /context slash command that opens a dialog (consistent with the existing /cost dialog) showing the current context-window composition, broken down by category.

Motivation

Competitors (Claude Code's /context grid) let users see exactly what is consuming their context window. docker-agent today shows only a single aggregate "Context: N%" in the sidebar and a cost-centric /cost dialog — there is no window-centric breakdown.

Proposed behavior

Show estimated tokens and % of the resolved context limit for each category:

• system prompt
• tool definitions
• prompt files (AGENTS.md / add_prompt_files)
• conversation messages
• tool results
• compaction summary

Render as a dialog with a bar/grid. Numbers are estimates (len/4 heuristic via compaction.EstimateMessageTokens) — label them as such.

Implementation notes (starting points)

• New command in pkg/tui/commands/commands.go
• New message type in pkg/tui/messages/session.go
• New dialog pkg/tui/dialog/context.go
• Breakdown computed in pkg/runtime from Session.GetMessages output; consider a ContextBreakdownEvent
• Reuse pkg/tui/components/toolcommon/tokencount.go

Complexity: M. Command name decided as /context (not /status).

Background: shared plan context-management-gap-analysis (Phase 1, item 1).

Dependencies / Relationships

• Blocks: feat(tui): context inventory — list and remove attached/prompt files (/drop) #3435 (context inventory extends this /context dialog), feat(runtime): cross-agent compaction orchestration (explicit, user-triggered) #3439 (team-level view extends /context)
• Related: feat(tui): per-sub-agent context accounting in the TUI #3438 (reuses breakdown computation), feat(compaction): reconcile token estimator with provider-reported usage #3437 (estimator accuracy improves the numbers shown here)