feat(site/src/pages/AgentsPage): group context indicator resources by directory

[kylecarbs]

What

Groups the Skills and Context files lists in the context-usage popover (ContextUsageIndicator) by their parent directory. Resources pulled from different roots, for example a repo-root AGENTS.md and a nested site/AGENTS.md, or same-named skills from .coder/skills vs .agents/skills, previously collapsed to identical basenames and lost their provenance. Each group now renders a dimmed directory header with a folder icon, and its items are indented beneath it.

Why

ChatContextResource.source carries the real path for instruction files and skill directories, but the popover only rendered the basename, so multiple context roots were indistinguishable.

Notes

• The directory header shows even for a single root, so provenance is always visible.
• MCP is intentionally left flat for now: an mcp_server's source is a server name (not a path) and the row carries no reference to the mcp_config that declared it, so it does not fit the directory model without a backend change.
• Adds a getPathDirname helper (with tests) and switches the skill list key from skill.name to skill.source to avoid collisions across roots.

Test plan

• ContextUsageIndicator.stories.tsx: new MultipleContextRoots story (files + skills across several directories); Clean story updated to assert the single-root header. Storybook play tests pass (4/4).
• path.test.ts: added getPathDirname cases (unit pass).
• biome check, tsc -p ., and make pre-commit all pass.

Design decision log

• Header gating: started by only showing the directory header when a section spanned more than one directory; changed to always show because the provenance is useful even for a single root.
• Presentation: chose a grouped layout (dimmed directory header + folder icon, items indented beneath) over an inline dimmed path prefix, since there is typically a lot of shared structure to convey.
• Directory labels currently render the full path (dimmed, truncated with a title tooltip). Open follow-ups: collapse $HOME to ~ or show fewer path segments if the labels feel long.
• MCP: evaluated applying the same grouping. An mcp_server is keyed by name and has no link to its mcp_config (confirmed in codersdk.ChatContextResource and the pinnedContextResources builder), so grouping servers under a config would require a backend data change. Left flat for this PR.

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🤖 This PR was created by Coder Agents on behalf of @kylecarbs.