Improve TUI sidebar session insights

[alejandro-ao]

Summary

• replace redundant provider/model/thinking sidebar metadata with the session name and active-branch activity totals
• show cumulative input/output usage, estimated model cost ($N/A when unavailable), automatic-compaction status, and loaded extensions
• render tools, prompts, and extensions as compact comma-separated lists while keeping skills and context paths as bullets
• widen and space the borderless sidebar, use bright headings with muted values, match the prompt background across themes, and pin the Tau version mark to the bottom
• remove Textual's top header and shortcut footer to recover vertical space while retaining dynamic terminal titles and keyboard shortcuts
• emphasize the unlabeled session name and working-directory basename, and place provider:model (thinking) above used/limit in the compact status block
• simplify the prompt editor to a single state-aware left border while preserving its original padded size and background block
• use the prompt/sidebar background with light vertical padding for user transcript messages in every built-in theme
• constrain Markdown hover underlines to the exact linked text
• promote the dark aqua highlight to the global accent and derive autocomplete selection styles from the same palette used by modal lists

User experience

The sidebar now focuses on durable session identity, capabilities, and lifetime branch usage instead of repeating the compact model status line. Its wider, more clearly separated sections expose useful cost and activity information, preserve full external context paths, and retain Tau's identity and version without consuming global header or footer rows.

Issue

No linked issue. This addresses the agreed TUI sidebar improvements from design discussion.

Manual validation

1. Start tau in a wide, tall terminal and confirm there is no top header or shortcut footer.
2. Confirm the sidebar is wider, matches the prompt background, and shows τ = 2π with the current Tau version at its bottom edge.
3. Name a session and confirm the name appears in the sidebar and terminal tab.
4. Run several prompts that invoke tools and confirm turns, tool calls, tokens, and estimated cost update.
5. Run /compact and confirm lifetime activity/usage totals remain while current context usage shrinks.
6. Confirm skills and context files use bullets while tools, prompts, and extensions use comma-separated lists.
7. Resize the terminal until the sidebar hides and confirm the terminal tab still identifies the session.

Validation

• uv run pytest (991 passed)
• uv run ruff check .
• uv run ruff format --check .
• uv run mypy
• uv build
• cd website && hugo --minify

The optional local Pagefind build could not download pagefind because the configured npm registry timed out; GitHub CI installs it independently.