Shape clipboard content for its recipient; add Outlook target

[Jodre11]

When we share a Claude response with a colleague, two things go wrong often enough to be
annoying: it gets pasted into Outlook in the wrong format and has to be re-requested, and it
carries context that is accurate but irrelevant to the person reading it, which is the main
reason colleagues stop reading these messages at all. This PR closes both gaps in the
md-to-clipboard plugin — it asks where the message is going and who it is for, then shapes it
accordingly.

The three commits are separable and can be reviewed in order.

Changes

feat(md2clip): add --outlook target — Teams and Outlook need opposite HTML. Teams strips
<p> margins, so spacing has to be faked with blank lines, and it auto-borders tables. Outlook
honours <p> spacing and draws no gridlines on an unstyled table. --outlook keeps the <p>
elements and the real typography, adds the legacy border/cellspacing/cellpadding
attributes Word's renderer honours, and drops pandoc's <colgroup>, whose source-derived column
widths bear no relation to rendered content.

fix(md2clip): stop padding around headings and blockquotes — the Teams path rebuilds
paragraph spacing with blank lines, skipping elements that already carry their own margins.
h1–h6 and blockquote were missing from that exclusion list, so a heading got a blank line
on top of its own margin and a blockquote was padded both inside and outside. Both guards now
include them; plain paragraph gaps are unchanged.

feat(md-to-clipboard): shape content for its recipient — adds two gates before anything
reaches the clipboard. The channel (Teams or Outlook) is never guessed, since a wrong guess costs
a round trip. The recipient selects one of three tone tiers — engineer, manager, executive.
Both are inferred silently when the user has already stated them, and skipped entirely on a
verbatim copy. Selection is by whether a fact changes the recipient's decision, so length falls
out of the filter rather than being a target, and structure is judged the same way: comparative
tables and ordered steps stay, headings above single paragraphs go.

Test plan

• md2clip --outlook --debug retains <p>, preserves em dash and smart quotes, and emits
  border/cellspacing/cellpadding with border-collapse on the table
• md2clip --debug (Teams) still flattens <p> to blank lines and leaves <table> bare
• Headings and blockquotes no longer gain surrounding blank lines; the gap between two plain
  paragraphs is unchanged
• tests/run.sh structural checks pass (final newline, executable bit, indentation, README)
• Paste both targets into Teams and Outlook and confirm the rendering by eye

Note for the reviewer

tests/run.sh resolves REPO_ROOT with git rev-parse --show-toplevel, so it only passes when
invoked from inside this repo and fails immediately when run from another working directory. Not
addressed here, but worth a follow-up.