Deep-link anchors for issue and proposal headings

Background

MuseHub already renders anchor links on hover for headings in README files and mists: when you hover over a heading, an octothorpe (#) appears to the right. Clicking it updates the browser URL bar to #section-slug and produces a shareable deep-link into that specific section.

Issue and proposal bodies are rendered Markdown and support the same heading syntax, but the hover anchor behaviour is absent. This creates an inconsistency: the same ## Background heading in a README gets a deep-link, but the same heading inside an issue body does not.

For long implementation plans (phased issues, design docs, ADRs) the inability to link directly to a phase or section forces authors to add workarounds like HTML anchor tags or redundant section references.

Goal

When viewing any issue or proposal on MuseHub, hovering over a rendered Markdown heading causes an octothorpe to appear. Clicking the octothorpe:

1. Updates the browser URL bar to append #<slug> (e.g. #background, #phase-2).
2. Scrolls the page to that heading.
3. The resulting URL is a shareable deep-link that, when opened, scrolls directly to the heading.

The behaviour and slug generation must be identical to the existing README and mist implementation so that cross-document links work predictably.

Design

Slug generation

Follow the existing rule (already used for READMEs and mists):

1. Lowercase the heading text.
2. Replace spaces with hyphens.
3. Strip characters that are not alphanumeric, hyphens, or underscores.
4. Deduplicate: if the same slug appears more than once in the document, append -2, -3, etc.

Examples:

## Background            → #background
## Phase 2 — CLI         → #phase-2--cli
## `muse tag add`        → #muse-tag-add

URL shape

The anchor should be appended to the canonical issue or proposal URL:

https://staging.musehub.ai/gabriel/muse/issues/56#phase-2--cli

Navigating to this URL must scroll the rendered body to the matching heading without a full page reload.

Hover UX

Match the existing README and mist behaviour exactly:

• Octothorpe appears to the right of the heading text on hover.
• Cursor changes to pointer when hovering the octothorpe.
• Clicking copies/sets the anchor URL (same as GitHub behaviour).

Deliverables

• [ ] Anchor slug generation applied to issue and proposal rendered Markdown — same algorithm as README/mist renderer.
• [ ] Hover octothorpe rendered for h1–h4 headings in issue and proposal bodies.
• [ ] Clicking the octothorpe updates window.location.hash and the browser URL bar.
• [ ] Direct navigation to a #slug URL scrolls to and highlights the target heading.
• [ ] Slug deduplication: repeated headings get -2, -3 suffixes.
• [ ] Regression test: existing README and mist anchor behaviour unchanged.

Out of scope

• Table of contents auto-generation — separate feature.
• Anchor links in comments — can follow in a later issue.
• Cross-issue deep links (#56#background) — deferred.