feat: per-tool inline filter stats in CLI chat

[bug-ops]

Summary

• Add raw/filtered line counts to FilterResult and FilterStats
• Display inline filter savings after tool output in CLI: [shell] cargo test (342 lines -> 28 lines, 91.8% filtered)
• Update CHANGELOG.md, README.md, and mdBook docs for M26/M26.1 features

Changes

Core (4 files):

• filter/mod.rs — raw_lines/filtered_lines on FilterResult, count_lines() helper
• executor.rs — line counts on FilterStats, format_inline() method
• shell.rs — propagate line counts, move ToolEvent::Completed after filtering
• streaming.rs — display inline stats when filtering reduced output

Docs (6 files):

• CHANGELOG.md — entries for feat: token savings tracking and metrics (M26) #445, TUI: resources/context/session metrics not updating after initial load #447, feat: display filter metrics in TUI dashboard #448, feat: display per-tool output filtering stats in chat and TUI #449, feat: output filter architecture improvements (M26.1) #452, refactor: remove 8 lightweight feature gates, make always-on (M26) #438
• README.md — filter system, always-on features
• docs/src/guide/tools.md — output filter pipeline section
• docs/src/guide/tui.md — filter metrics in TUI
• docs/src/feature-flags.md — always-on vs optional restructure
• docs/src/architecture/token-efficiency.md — filter pipeline and savings tracking

Test plan

• 1556 tests pass, 4 new tests added
• Clippy clean
• Fmt clean
• Manual: run CLI, execute shell commands, verify inline stats appear

Closes #449