# 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.