fix(terminal): paste multiline as LF in bracketed paste (PowerShell whitespace)

[openwong2kim]

Summary

Fixes the multiline-paste-whitespace bug in PowerShell panes. Two commits:

• Paste fix (clipboardChunk.ts): the bracketed-paste body now uses LF as the in-body line separator instead of a lone CR, plus ESC sanitization to block paste-injection.
• Docs (README.md): drop the removed Ctrl+Up/Down scroll-bookmark jump shortcut.

Root cause

Right-click / Ctrl+V paste of a multiline command into a PowerShell pane injected stray whitespace and split the command line-by-line. normalizePasteText collapsed every newline to a lone CR, but inside a bracketed-paste body PSReadLine treats CR as Enter and misplaces the cursor (PSReadLine #3939, #417 -- both recommend LF as the in-body separator).

Fix

• normalizePasteText(text, bracketed): LF when bracketed (the body is inserted, not executed), CR otherwise (each line is an Enter). Non-bracketed path unchanged.
• pastePtyChunked sanitizes a raw ESC in the bracketed body to U+241B so a pasted close marker cannot be forged to run trailing bytes as a command (paste injection).
• chunkOnDataIfNeeded is covered automatically (it delegates to pastePtyChunked), so all four paste paths (Ctrl+V, Ctrl+Shift+V, right-click, onData) are fixed at once.
• Intentionally diverges from xterm.js (which normalizes to CR in both modes); wmux bypasses xterm''s paste pipeline and owns this choice.

Test Coverage

• clipboardChunk.test.ts: +16 tests (bracketed to LF separators, ESC-injection guard, non-bracketed CR unchanged, empty/edge). 35 total in the file.
• Full suite: 2057 pass, tsc --noEmit clean.
• Dynamic repro (scripts/paste-repro.cjs, real pwsh 7.6 / PSReadLine 2.4.5): a CR body splits the command, an LF body buffers it as one multiline command.

Review

• /plan-eng-review: 2 decisions locked (separator policy = bracketed->LF; ESC sanitize folded in). Design notes in plans/paste-newline-fix.md.
• GUI dogfood: confirmed by maintainer -- multiline right-click paste into a PowerShell pane no longer injects whitespace.

Test plan

• clipboardChunk 35 tests pass
• Full vitest suite 2057 pass
• tsc --noEmit clean
• GUI dogfood: multiline paste into PowerShell pane

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