auto-task(Capacity): clear QuantumInfo/Capacity/Capacity.lean from the linter exemption list

[doxtor6]

Summary

Removes QuantumInfo/Capacity/Capacity.lean from scripts/LinterExemption.txt, so the CI
linters (lake exe runPhyslibLinters and ./scripts/lint-style.sh) now enforce their rules on
it, and fixes the file so it passes them.

No open PR was handling this file (in particular #1244, which clears 56 other QuantumInfo
files, does not touch it).

Changes to QuantumInfo/Capacity/Capacity.lean

All changes are style/documentation only — no statement or proof was changed in meaning:

• Reflowed all lines to the 100-character limit (doc-strings, theorem signatures, and a few
  proof-internal line breaks), using the standard 4-space continuation indent for split
  signatures.
• Restructured the module docstring into the standard Physlib layout
  (i. Overview / ii. Key results / iii. Table of contents / iv. References), keeping
  the original physics content: the emulation-based definition of quantum capacity, the basic
  facts proved here, and the LSD (Lloyd–Shor–Devetak) theorem statements.
• Filled in the table of contents and added matching lettered section headings
  (A. Basic definitions through E. The quantum capacity) so the file also passes the
  optional lake exe module_doc_lint.
• Added the missing doc-string on quantumCapacity (the supremum of achievable transmission
  rates), required by the docBlame check in runPhyslibLinters.
• Split an overlong variable line.

Verification

• lake build succeeds (9119 jobs).
• lake exe runPhyslibLinters reports no issues for this file (remaining local failures are
  all in files still on the exemption list).
• ./scripts/lint-style.py QuantumInfo/Capacity/Capacity.lean exits clean, and
  ./scripts/lint-style.sh reports no new issues.
• Optional linters: module_doc_lint and spelling also pass for this file.

🤖 Generated with Claude Code

[github-actions]

Thank you for this PR, which will now be reviewed. If submitting to ./Physlib or ./QuantumInfo, please see our review guidelines if you are not familiar with the process. You should expect a back and forth with a reviewer before your PR is merged. See also that link for how to add appropriate labels to your PR. The PR will also go through a number of automated checks. You can learn more about these here, including how to run them locally.

If you are submitting to ./PhyslibAlpha there will be a lighter review process, though your PR must still pass the automated checks.

If you want to bring attention to this PR, please write a message on this thread of the Lean Zulip.

Important: If a reviewer adds an awaiting-author label to your PR, once you have addressed the review comments, please remove that label by adding a comment with -awaiting-author. This helps us keep track of reviews.

[jstoobysmith]

Approved - will merge once passes linters (removed an entry from the API-map).